There are a number of ways to make money in World of Warcraft. The most important reason to make money in the endgame is to gain access to your epic flying mount, which costs a total of 5,200gp – 5,000g for the training and 200g for your mount. After that, the only major expense you will face is if you decide to buy epic items on the auction hall. It isn't too difficult to reach this amount – all it takes is time and focus.
Questing
The first way to make money is to complete quests. Once you reach level 70, the Outlands quests tend to reward 10 or more gold for turnin, in addition to any items and vendor trash you pick up off of enemies. In addition, if these are done before you begin to collect a lot of gear from dungeons, you can pick up some free equipment upgrades from the quest rewards. As long as you also use your gathering skills at the same time, you can easily make a great deal of money in a short amount of time. The other side of questing to make money is daily repeatable quests from the Skyguard and Ogri'la factions. These quest do not take a great deal of time to complete, although the Ogri'la faction does have several boss-level quest encounters to begin their quest chains. Both these factions do require you to have your regular flying mount, but as long as you save your money while you are leveling, this should not be a major problem. Completing the daily quests give about 10g per quest, and you can complete up to 10 daily quests a day. Realistically though, until you have access to the Netherwing daily quests (which require an epic flying mount), you won't have ten daily quests available each day.
Gathering Skills
The second way to make money is through your gathering skills. If you have skinning, mining, or herbalism, the materials you collect tend to end up being worth a fair amount on the auction house to people who don't want to spend the time collecting these resources, or find themselves short on one or another. While you're leveling, don't go out of your way to spend a lot of time running around trying to find mines and herbs... but if you see them on your minimap, feel free to make a detour to pick them up. WoW tends to stick at least one enemy peacefully

admiring the shine of the ore or the serenity of the flower you want to pick, so it's a good way to gain some extra experience while also making a bit of money. Once you have a stack of material, look on the auction hall and sell your materials for a little under the lowest price for another full stack. After a while you'll get a sense of how much certain materials are worth, so if you see the price is much lower then you expected, hold on to your materials and sell them another time.
For finding materials, most zones have their own advantages, and many zones also have enemies that can be farmed for motes of mana, fire, life and so on, which are also a good source of income, especially if you do not possess other gathering skills. You can generally farm 4-8 primals an hour, depending on the area, but generally you should be able to make 100-120g an hour farming primals, and a similar amount when collecting minerals or herbs.
Hellfire Peninsula
Mining: Fel Iron deposits, rare Khorium deposits run along the mountains that border the edges of the zone, be careful in the northwest corner where elite mobs roam. Khorium sometimes spawns in place of other minerals, and requires max mining skill (375) to mine. It's very profitable if you do not require it yourself, and goes for about 5g per unit.
Herbalism: Felweed, Dreaming Glory, Golden Sansam, Dreamfoil, Mountain Silversage. Like mining, check along the mountain edges for most of these herbs, and also check in the desert basin to the southwest of Honor Hold.
Primal Shadow: The void walkers that spawn in the southeast of the zone can drop Motes of Shadow, and are easy to farm to make Primal Shadow, although the drop rate is somewhat low.

Zangermarsh
Mining: Fel Iron, Adamantite, Khorium. Fel Iron is more common then Adamantite in this zone, but staying along the zone edges near the mountains is the fastest way to find minerals here. Deposits might also show up near Coilfang Reservoir, but are inconvenient to reach due to the swimming requirements.
Herbalism: Felweed, Dreaming Glory, Ragveil, Blindweed, Flame Cap. The entire swamp contains locations for these herbs, so there's no one path that nets you the most herbs. Despite the looks of it, don't ignore the Dead Mire in the northeast however – it contains a number of herb locations, despite its blasted appearance. Also, don't mind fighting the bog giants that roam in the southwest and northeast, since they can be herbed and occasionally produce motes of life from doing so.
Primal Life: Even if you aren't an herbalist, the bog giants in the southwest Spawning Glen or the northeast Dead Mire have a chance to drop Motes of Life when slain.
Terokkar Forest
Mining: Fel Iron, Adamantite, Khorium. As always, run along the edges of the zone and keep an eye on your radar. Fel Iron tends to spawn in the south and north where the land gives way to the twisting nether, but can also be found elsewhere. Adamantite is more often found near the mountain ranges.
Herbalism: Felweed, Dreaming Glory, Terocone, Mana Thistle. Felweed can be found everywhere in the zone, while Dreaming Glory is found near the mountains and Terocone exclusively in the green areas of the map. There's no one place that's perfect in this zone, so wander around and see what you can find. Mana Thistle can only be found with a flying mount on the plateau to the northwest of Shattrath or in Skettis, to the southeast corner of the zone. Both areas have level 70+ enemies, including elites, so be careful when attempting to farm this herb. Skettis also has flying enemies who can attack you, so make sure to keep an eye out above you as well as around you.
Primal Water: Directly to the east of Shattrath, the lake here is a great place to farm motes of water- the fish are easy to defeat and have a high respawn rate, making them perfect for farming.

Nagrand
Mining: Fel Iron, Adamantite, Rich Adamantite, Khorium. Search the valleys in the center of this zone, as they have the best chances of having spawn points, but also look at the mountains to the north and east, which have a fair chance of mining spots. Rich Adamantite veins sometimes spawn instead of regular ones, and can be mined twice as much, while also tending to provide more motes of earth.
Herbalism: Felweed, Dreaming Glory, Mana Thistle. Nagrand doesn't provide any unique herbs, so search near the mountains for Dreaming Glory, and in the green plains for Felweed. Often you can look elsewhere and pick up some rarer herbs at the same time, however. Mana Thistle can possibly be found to the far west of the zone, past the Forge Camps. There is a path up to this secluded valley you can travel on foot, but with patrolling elites (including a quest boss) in that area, it's better to wait until you have a flying mount and see if you can swoop in, recover the herb, and fly out rather then try on foot.
Skinning: The clefthoof bulls that wander through the center and northern area of the zone have a higher chance to drop Clefthoof Leather, which is required for Clefthide Armor Kits. The secluded vale in the far west is home to various snakes who drop Cobra Scales, another somewhat rare item required for various leatherworking recipes.
Primal Earth, Air, Fire and Water: Many earth and air elementals roam the plains of Nagrand, but there is also an area in the northeast that can only be reached by a flying mount that contains level 72 versions of most elementals. It's heavily farmed, so you might have trouble getting your own kills, but it does contain plenty of elementals of all types.

Blade's Edge Mountains
Mining: Fel Iron, Admanatite, Khorium. Blade's Edge Mountains doesn't provide the best environment for mindlessly gathering resources. Portions of the map are inaccessible except by flying mount, and the entire zone is sub-divided by the mountain ranges into smaller zones. Due to the number of enemies you encounter in this zone, and the zone layout, grinding for resources is too inefficient to recommend here – it's better to stay in a more open zone such as Nagrand or Terokkar Forest. One note is that after you complete the quests to allow you to begin Ogri'la quests, many of the ogre tribes become neutral to you – which makes this one of the safer zones to mine in, if you stick to their parts of the zone at least.
Herbalism: Felweed, Dreaming Glory, Mana Thistle: Like with mining, the zone layout makes it difficult to herb efficiently in this zone. Like with mining, if the ogres are neutral it's one of the safest places to pick up Felweed. Mana Thistle can be found in the forge camps on the western plateau, or among the crystals of the northern plateau, which is a better place to look for it. Both of these areas require a flying mount to reach, however.
Skinning: The netherwing dragons in the southeast crystal fields drop Netherscales, for Dragonscale leatherworking.
Primal Fire: To the north of Toshley Station is a scorched area, filled with fire elementals and imps. Killing the elementals tends to yield Motes of Fire, making it a good place to farm for Primal Fire.
Primal Shadow: On the western plateau, the imps and other demons of the southern Legion camp have a fair chance of dropping motes of shadow. They're equal level, so they aren't the easiest enemies to farm, but they do have a better chance of dropping motes of shadow then do the void walkers in Hellfire Peninsula.

Netherstorm
Mining: Fel Iron, Adamantite, Rich Adamantite, Khorium. Like Blade's Edge, the layout of this zone makes it difficult to grind mines efficiently without a fast flying mount. Adamantite is a lot more prevalent here then Fel Iron, since Fel Iron is mostly found in the Legion camps on the northern island. Search along the edges of the islands, but be careful due to the number of enemies that exist along these areas, including powerful giants and mana-draining creatures.
Herbalism: Dreaming Glory, Mana Thistle, Netherbloom. Netherbloom is the primary crop of this zone, since it is unique to Netherstorm. Dreaming Glory can be found here and there along the ridges that dot the islands, while Netherbloom is found mostly along the edges of the islands. Mana Thistle can be found on the small floating island above the area in the northwest, but again caution is warranted, due to the presence of Socrethar, a difficult 5-man quest boss. Still, you should be able to herb without making friends with him, and the other demons on the island aren't too difficult.
Skinning: On the eastern island, the dragons in the crystal area are a prime spot to farm Netherscales.
Primal Mana: The mana-draining fish that float in from the void tend to drop Motes of Mana, and can be farmed for them. This is a difficult task on a mana-using class, but it can still be done, though you will have to drink more often.

Shadowmoon Valley
Mining: Fel Iron, Adamantite, Rich Adamantite, Khorium, Nethercite. Fel Iron can be found here and there near the lava flows and the Legion camps. Adamantite has a nice cluster of possible points to the northwest of the Altar of the Sha'tar. Nethercite can be found at Netherwing Ledge, and is used for faction with Netherwing. The nice thing about Nethercite deposits is that they also have a good chance to contain motes of earth and fire. However, without having started the Netherwing quests at Netherwing Ledge, you will be hostile to the 3-5 peons nearby, and therefore these are fairly difficult to get to before you've acquired your epic mount. Also be aware of the roving elites, who remain hostile even when you appear to be a fel orc.
Herbalism: Felweed, Dreaming Glory, Mana Thistle, Nightmare Vine, Netherdust Bush. Nightmare Vine happens to be the unique herb of this zone, and can be found almost everywhere, though the area northeast of the volcano in the center of the zone has many possible spawning locations. Felweed is also possible to find just about anywhere, while Dreaming Glory is found around the volcano for the most part. Mana Thistle and Netherdust Bushes can be found at Netherwing Ledge, though the same warning applies when all enemies are still hostile. However, the herbs tend not to spawn directly next to peons, but instead in the middle of the various paths in Netherwing Ledge, so they are more collectible. Netherdust bushes occasionally drop Motes of Mana in addition to Netherdust Pollen, which is turned in for Netherwing faction.
Primal Air: The air elementals to the south of the crystal fields in the southeast corner are good to farm for motes of air – they're found all along the ledge that runs along the edge of the void.

Auction House
Make sure that you try to sell most green items on the Auction House first The exception to this of items of Spirit – while you can find buyers for Strength, Stamina, Agility and Intellect, very few people will take a look at items that provide only a Spirit bonus. Items that provide odd combinations such as of the Wolf (Agility and Spirit) also tend not to sell well, and can be safely vendored or disenchanted without attempting to sell it on the Auction House.
For everything else, it's worth making at least one attempt to sell your green items on the auction house. The first step is to look in two or three levels above and below the level of your item in its category – for example, you'd want to look at plate helmets in a level range of 48 to 52 if you have a helmet that requires level 50 to use. Take a look at what other people are selling their items for and price accordingly to be just under a similar green item.
Blue items are an exception – here it's a good idea to look at a database like Wowhead and find out roughly how much your item should be going for – if it sells for a too little compared to that price, make a decision – usually waiting until the ones being sold too cheaply are off the market doesn't take more then a day or so. If you are lucky enough to find a world drop purple item and you decide not to use it, spend a moment finding out how much it should sell for, and don't be discouraged if it takes a few tries. Items like the Ancient Scepter of Sue-Min can take a couple tries to sell before someone with the money to buy it will come along and take it. Once it does sell, you'll be much closer to buying your epic flying mount.

You can also turn your trade skills to make a profit. However, this is where you have to be careful in what you make. Service skills, like enchanting and jewelcrafting have the least chance of losing money – someone asks you for a type of cut for a gem or an enchant, and if you have it, you make a profit from their tip. However, this does have a great deal of time overhead, both in collecting the enchants or jewelcrafting patterns (Which generally require visiting dungeons and hoping for a very rare drop for the best ones), as well as advertising your skills in the trade channels in a city like Shattrath.
Other skills, such as leatherworking or tailoring can also make a profit, but before you go to create an item for sale on the auction hall, take a look at what that type of item is currently going for, and price out what it would cost to make the item if you had bought all the materials off of the auction hall. If the number you calculate is higher then what you want to sell your item for, it's a better idea to sell the materials themselves to other crafters and make a higher profit that way.
The final way to make money with your trade skill is if you happen to have hard to find patterns – as long as others are willing to collect the materials, you can ask for a tip and create the item for them, saving you the time and effort of collecting materials.

Final Thoughts
The most important tip for seeking to buy something is to remember your goal. When you reach level 70, you'll be pulled in several directions – wanting to max out your trade skills, pick up new gear now that it has become available, and working towards your epic mount. Whatever you do, pick one of these goals and work towards it without being sidetracked. I've seen a number of players wishing they had an epic flying mount, but when I ask they what they've been spending their money on instead, it turns out they were picking up this really great item on the auction hall, it only took most of their money.... you get the idea. If you want the epic mount, don't let that blue or purple item on the Auction Hall distract you and don't spend money on things that you don't absolutely need. Otherwise, you'll never get enough funds together to buy your flying mount.
Once you've picked up an epic flying mount, everything else becomes much easier – dailies are much quicker to complete, and the Netherwing daily quests become available to you as well. Finding resources also goes by much quicker, as you zip around zones nearly 300% faster then on your epic land mount, and much faster then on the first flying mount. This allows you to make more money in less time, making achieving the rest of your goals that much faster.
To summarize, the fastest way to make money is through questing after you've reached level 70. Through completing dailies and any of the quests you did not do (and at least one zone should be untouched by the time you reach 70), you can make a great deal of gold. The second best place to make money is through the auction haul – through selling green items, cloth and other materials for crafters to use. While finding a purple item will definitely boost you well on your way to buying an epic flying mount, enough time and patience will get you there as well.