Strengths & Weaknesses
Mages are the undisputable kings of burst damage. They have the worst armour and generally very low hitpoints, so rely on killing quickly. They also have a range of abilities which prevent damage by preventing their opponent from attacking. There are two major PvP builds – the frost mage and the fire mage.
Frost Mage
The frost mage is a high-survivability, moderate damage build. There are a number of tricks which allows the frost mage to survive combat for a much longer period than other mage builds, which is important because of the low base survivability that mages have.
This survivability comes from too major areas: talents which unlock abilities that other mages don’t have access to, and kiting. The two key surviving abilities that frost mages have are Ice Barrier and Ice Block. Ice Barrier absorbs a set amount of damage, and is on a 30 second cooldown (it lasts one minute, which means that if you time your application correctly, you can re-apply it immediately). Ice Block is basically immunity, but with the detriment of preventing any action from the mage.
When using Ice Block, it is important to remember that you can remove the buff yourself whenever you want. You don’t have to wait the entire duration – if you right click on the buff, it will instantly remove it, and you are free to make any action. This makes it excellent for removing debuffs and returning to combat, or waiting temporarily for backup. It also confuses the opposition if you break early. It’s usually best to Blink as soon as you remove the buff.

Other than those two abilities, Frost Mages are excellent kiters. This is the process of killing your opponent while preventing them from attacking you, through slowing spells and use of Blink/running. Cone of Cold, which has a long snare duration, and is insta-cast, is a key to effective snaring, and Frostbolt will be the majority of you damage in a 1v1 situation.
Frost mages are also great in group PvP, especially if they have Improved Blizzard. In AV, they can slow an entire push very effectively for a low mana cost. The key to this is to not cast the max rank Blizzard (as most people aren’t stupid enough to stay in the AoE for the entire duration), but rather to cast Rank 1, as it still contains the slow effect (assuming you have the talent for it), and costs much less mana.
Fire Mage
The fire mage has a greater capability for burst damage, but lower survivability than a frost mage. Fireball does greater damage than Frostbolt, and Ignite makes up a large portion of DpS which frost builds lack.
The key to fire mage success is gear and Presence of Mind. Damage boosting trinkets, like the Zandalian Hero Charm, provide a massive boost to power, and should be used in combination with Fireball/Pyroblast and Presence of Mind. Because of the long cast time of Fireball (and to a greater extent, Pyroblast), it is rare that you will have the opportunity to cast without someone noticing you and attempting to interrupt your spells.
However, this is not always the case, if you can position yourself. Use the terrain to your advantage as much as possible. This is especially the case in Warsong Gulch, where the tree stumps make effective hiding tools.
If you go the Combustion route, a combusted Blast Wave is an excellent combo that can deal out significant damage to a large number of people. This is particularly so in places like AV, where lots of people are congregated. The snare element of Blast Wave is not be understated, also, as it allows some much-needed survivability that a fire mage largely lacks otherwise.

General Strategies
A key element of mage PvP is Counterspell, and you need to know how to use it and when to use it. If you have Improved Counterspell (which is less and less common these days), you have 4 seconds of global silence, regardless of when you cast it. If, however, you just have the generic Counterspell, you get 10 seconds of silence for one school only, and only if you interrupt a spell being cast. For this reason, it is pointless casting it preventatively; you need to cast it as a reaction.
Timing is the key. If you miss-time your cast, your chances of success are significantly reduced. This is timing in the sense of in the middle of a cast. However, it is also important to know you opponent to know when in the battle is the most effective time to Counterspell. Know your opponent and their talent spec (which you won’t know immediately, but will gather as you battle. For example, a priest in shadowform will rely only on shadow spells, while one out will be casting smite. In the first case, it is easy – counterspell anything. In the second, you will need to counterspell a holy spell, to prevent them from healing).
Green/yellow glowing hands are healing spells generally. Black is shadow (warlocks/priests), while red is fire (mages/warlocks) and blue is frost (other mages). Counterspell is extremely powerful – so much so that it is currently the only way to prevent invulnerability spells, which normally work through anything. If you can Silence a paladin’s holy line, you can prevent him from using Divine Shield (the same goes for a mage’s frost line and Ice Block, which is easier). Additionally, if you have Improved Counterspell, you can use it on warriors to prevent them charging into combat, or on rogues to prevent vanish.

Blink is the other key component of mage pvp. Blink not only moves you instantly, it also removes stuns, so is very useful against rogues (and the occasional warrior or Unstoppable Force user). For this reason, you need to be conservative with its use. Don’t use it when it is up, just for the sake of it (unless you are running between battles, but even then it’s probably better to wait and mount up).

If someone is charging you, or a melee class is on top of you, use Frost Nova and run forwards in the opposite direction, don’t back up. Using Blink here is a waste of a cooldown. It is much more useful to save it until they have caught up to you again.
Macros
Trinket + PoM + A Spell
This uses the first trinket slot item, cast presence of mind, and casts a spell of your choice. Shift+click on the spell to insert it into the macro.
/script UseInventoryItem(13)
/cast Presence of Mind
/cast “Spellname”*
*Insert your spell of choice here, whether it be pyroblast, polymorph or frostbolt, or something else altogether.
Arena
You have two major options here – burst damage, or survivability. As entering an arena will reset all your cooldowns of less than 10 minutes, all your tricks will be at your disposal every time you enter an arena. This means Ice Block, Cold Snap, Water Elemental etc. can all be used every time you play.
For this reason, Frost mages are very powerful. They can put out quite good DPS, but most importantly have the best survivability of all the cloth wearers, and if they are being focused can Ice Block which really throws off the other team. The duration of Ice Block will allow a healer to return the mage to full health. Dual Iceblock, using Cold Snap, is also effective (although this will be limited to once every 30 seconds with the new “Hypothermia” debuff. Water Elementals can also harass and provide good damage, or Nova melee classes in place.
Fire mages have a bigger damage potential but will die much faster, and will most likely be targeted first. PoM + Pyroblast has huge damage potential, particularly if it crits, and with good team co-ordination you can possibly kill one player before they can react.
Arcane builds simply don’t have the damage potential or the survivability of the other 2 specs. There is potential for use of Slow in 2v2s, where having a dispeller available is less likely. However, with the popularity of paladins, it is likely you will come across a dispeller, and Slow will be removed almost immediately.

Spellsteal is a very important tool in arenas. Against teams with a paladin, Blessing of Sacrifice will be used to prevent you sheeping the paladin. In this case, you need to use Detect Magic to determine who has that Blessing on them. Not only will tis allow you to sheep the paladin without fear of it breaking, you will gain a very nice buff.