The warlock is a versatile class, and can deal with many different situations effectively by him or herself. First off, a few tips and strategies:
Maximum Casting Range
Once you've played your warlock a while, you should learn to take maximum casting distance when attacking an enemy. This is a principle that holds true for most classes with ranged attacks. You can tell whether you are at max range by looking at your action bar. When the number corresponding to the skill turns red, you are out of range. Edge closer until it turns white, and you'll be at max range. You will get a feel for this as you spend more time playing a ranged attacker. Once an enemy starts hitting you it will cause delays in your spells’ cast time. Any hit incurred will increase the time it takes to finish the spell you’re casting, with diminishing returns for each hit.
For example, the first hit an enemy lands on you increase the cast time by 1 sec, the second hit will only bump it by 0.8 sec, the third hit by 0.6, and the fifth hit by 0.4 sec. (figures not based on the actual numbers) It never reaches 0 though. This makes it still possible to get a spell off if you are being hit by two, even three enemies with some luck (and a big enough healthpool), but the added delay does hinder you a lot. Any more enemies hitting you, and you’ll have a very hard time casting anything other than instant spells.
If you are using a channelling spell and take a hit, the duration of the spell will decrease by about 30%. The Fel Concentration talent will give you an up to 70% chance to counter this effect for affliction based channeled spells, the same hold true for Intensity and Destruction spells.
Curses!
When soloing I find my most uses curses to be:
-Curse of Shadow on a demon I plan to enslave, or just to have spells land easier, and to improve nuke damage.
-Curse of Tongues when fighting casters (at least on the ones that actually use non- instant spells).
-Curse of Agony, when I’m fighting something I know will take a bit to kill.

-Curse of Doom (level 60), on a tough target or on an add. Quite effective if the target lives long enough for it to do its job.
-Curse of Recklessness, when whatever I’m killing will try to flee. Tyically use rank 1 for this.
-Curse of Weakness can be a helpful when you dont have a warrior tank, and the mobs are using attack power based abilities. Mobs do not have much attack power, and any ability of theirs that relies on it will be significantly hampered by even small reductions in AP.
10% Aggro
In order for you to steal aggro from your pet, or your pet from you, the difference in threat has to be over 10%. Because of this, it can be difficult for a pet to take aggro from you, especially later in a fight. (when that 10% gets hard to overcome, as it applies on the threat accumulated in the entire fight). If you want to avoid taking too many hits, at least let your pet get in a few hits before opening fire.
Mana Efficiency
Warlocks are a quite mana efficient class, and we have our means to get more mana quite easily. However, this does not mean you should waste your mana. The most common practise of mana wasting comes in the form of overdotting. Overdotting is what I call putting a 24 second DoT on a target that will not last for more than 10 seconds. Do yourself a favor and see wheter it is a good idea to be fighting your targets the way you are. Maybe you would kill those targets if you traded out CoA and Corruption with a shadowbolt, saving both mana and time? Ask yourself this question from time to time, and you will be able to kill more things in less time.
Fear And Soloing
Fear can be a a great asset and save your life, but it can also be a great risk. If you plan on using it, make sure you are aware of that. If you are in an area thickly populated by enemies, fear can easily get you killed. The enemy running from fear will run past his buddies, which will cause them to aggro you.

You need some clear space in order to use fear without a high risk of drawing aggro from more enemies. Sometimes, a feared creature of a given type will not aggro another creature type. Fearing a bear or wolf past a group of humans will not cause the humans to aggro. This does not always hold true, so be careful not to assume things too much here.
Ritual Of Summoning
Sooner or later, you may find perfect strangers asking you for a summon. Some warlocks will refuse this, and some will charge for it, which is not unreasonable either. You have to drop what you’re doing, and find two people and bother them to assist you. Remember that WoW has means of transportation that don’t rely on warlocks (or mages), and you are not obliged to cut 5 minutes off someone’s travelling time. It is entirely up to you to decide if you want to invest the time required to summon people when they ask for it.
Conjuring
You might receive random tells from people asking you for a healthstone. In my experience this doesn’t happen very often. Charge them or don’t, you’re not unlikely to get a donation either way.
Pets
How you fight depends on which pet you use. You should approach a fight differently when you decide to use one pet over the other. This gets even more important if you delve into the demonology tree.
Imp – Solo Tactics
If you just started a warlock, you will be able to get your first pet, the imp, through a quest given in your starting town/outpost. You can actually get the quest at level 1, but it will be a little harder to complete then. You could hold it off until you’ve gained a few levels. The quest itself is very simple. Follow the directions in your quest log and say hi to your first little demon.
Here's some tactics you can use with the imp when soloing:

Trading aggro
Standing twice your casting range away from an enemy, send your imp to attack. Once he has landed a 2-4 firebolts (depending on if you have the Improved Firebolt talent), the enemy will have reached him. He can’t take a beating for long, so this is where you pitch in. Cast immolate and follow up with a shadowbolt or searing pain to pull aggro away from the imp and have the enemy charging to you. While the enemy is moving to you, you should be able to get another nuke off and have some distance between you and enemy to spare afterwards.* Immediately follow up with fear, and finish with a few more nukes, or let the dots run their course and recast them later, recasting fear as necessary.
If you use nukes, you can kill enemies off relatively quickly using this tactic when they are equal to or below your level. If they are higher level, their hitpoints will be too high to finish them off quickly, and increase the chances of getting adds while they are running from fear. If you have Curse of Exhaustion (Affliction talent), it can be used to slow them down.
* It’s possible to keep the pingpong effect of the enemy running between you and your imp going, continuously trading aggro, but that will take some more practise.
Support damage
Let the enemy hit you, and take the brunt of the damage during the fight. You’ll be using imp as support damage. Make sure you have fire shield on you. Pull with corruption, add your curse of choice, immolate, and shadowbolt or drain life for the rest of the fight while imp helps you take it down. Use this when there are many enemies near and you don’t want to use fear due to risk of adds. This works well against enemy casters. Use curse of tongues (level 26) when fighting casters. As a whole, this is not the most efficient way of fighting. You are wasting mana and life, as drain life isn’t an efficient spell manawise.
Imp is usually not the best choice in a scenario where you cannot fear and are dealing with melee enemies.

Voidwalker – Solo Tactics
Voidwalkers have the highest amount of hitpoints of all pets, and are excellent tanks. You can fight two or more equal level enemies at once with this guy, though they will take longer to kill. He has very poor damage output, so you have to do most of the hurting. Curse of Weakness can be an asset here, allowing him to survive longer. Keep in mind though that curse of weakness is a relatively high aggro curse compared to some of the others, and will draw aggro onto you if the target has not been taunted yet. Don’t start laying on too many dots and nukes right after engaging to avoid pulling aggro from your pet. Once the enemy has switched to attacking you, it is much harder to get it to attack your pet again. Either stop casting entirely until it switches back to the pet, finish it off or fear it. Sometimes, especially when you are planning to kill a tough mob, it can be a good idea to let voidwalker use both suffering and torment at the start of a fight to get some good aggro. It will be less likely that the enemy will turn its attention toward you, and you can use your spells more freely at the start of the fight.
Regular tanking
Maintain maximum casting range, and have your pet engage the enemy. Once he’s cast torment, you cast agony/corruption (or both), and wait till your pet casts torment a second time. You will want to slow your damage output a little if torment is resisted, so pay attention to the combat log. Cast immolate, and re-DoT or nuke as the fight progresses.
Hellfire / Rain of fire support
The voidwalkers’ sacrifice ability allows you to finish a part of the hellfire or rain of fire spell without being interrupted. Run into or pull the enemies you intend to treat to a fiery death. The moment you reach them, use sacrifice and start casting immediately. The sacrifice shield will not hold long, but you can deal a fair amount of damage this way.
Even better, you could sacrifice your void before actually engaging the targets. Summon a new voidwalker, send it in to the enemies. Have him use suffering while you start hellfiring. The moment the still active shield wears off, sacrifice your second voidwalker for another one.
Note: The soul link talent does work on the damage you receive from hellfire.

Decoy
Some quests require you to kill a boss or certain special enemy. If you are faced with a crowd of enemies with your intended target hiding in the middle of them, despair not. Send in voidwalker, and make him cast suffering when he reaches them. All enemies will start tearing your pet apart. Move in close, target your intended prey, cast Howl of Terror, and follow up with Curse of Recklessness on your target.. You have 10 or 15 seconds (rank 1 or 2) before howl of terror wears off, so you will want to kill quickly. Let your VW grab aggro quickly with torment. Cast Corruption, Immolate, and start casting Shadowbolt or Searing Pain. Add Death Coil and Conflagrate or Shadowburn if you have it. You can sacrifice the voidwalker to buy yourself a little extra time. If you finish the enemy before the 15 secs from howl of terror are up, you can loot and run. If not.. well, there’s always tomorrow. (and, they will head for your voidwalker first. So you should have a few extra seconds)
Succubus - Solo Tactics
This used to be my pet of choice through most of my soloing, and still is for the pre-felguard levels. Some warlocks prefer the Voidwalker, because he provides safer fights. The Succubus however, if handled well, let’s you achieve and maintain a faster kill rate then the Voidwalker does. The Succubus is usually full mana, and rarely runs out unless you are using her seduce ability a few times in a row. If you have the Dark Pact talent, I definately recommend using this pet. You can drain all her mana and she will keep doing most of her damage. (Barring Lash of pain, but that only only accounts for a part of her damage output) If you find she runs out of mana a lot, you could consider turning off Lash of Pain. Keep in mind that this will lower her damage output a little, and also lowers her aggro when engaging a target. (If she deals a regular melee hit and a lash right at the start of a fight, she has a decent amount of aggro)
You can use her seduce ability to interrupt a spellcaster. (If you can actually get a seduce off before the target lets loose the spell)

Basic killing
Get to max casting range from the enemy, and send in succubus. Let her get a few hits in, and cast corruption and immolate. Wait a short time, land a shadowbolt and you should have aggro. Land another bolt as the enemy approaches you, and either kill it from melee range, or fear and nuke it.
Nuke – seduce - nuke
This isn’t very efficient manawise, but can be useful. (It is actually an old PvP tactic). You start off with casting a shadowbolt or soulfire. While the nuke is nearly finished casting, let your succubus cast seduction. The nuke will hit the target, and get seduced right after. Cast another shadowbolt, and as it hits, let succubus recast seduction. Repeat until the target is dead.
Felhunter – Solo Tactics
The Felhunters' hitpoints and damage output lie between the voidwalker and succubus, and he is sparingly useful for solo play. He can play a tank role effectively, but has problems keeping aggro. If you outaggro him, he won't be taking aggro back in a hurry. Voidwalker is much better at keeping and regaining aggro.
Basic killing
Take max casting range and send in felhunter. The tainted blood ability will significantly lower the damage he takes, moreso if you team it up with curse of weakness. Burn the enemy down with dots and nukes, but be careful not to outaggro your pet unless you want to tank the rest of the fight. You could also use fear if you get aggro. With his lower hitpoints and lesser taunt abilities you are usually better off using voidwalker or succubus.
Felguard – Solo Tactics
The Felguard is by far my favorite solo pet. He has the hitpoints and armor to take a beating combined with excellent damage output and aggro abilities. You dont have to limit your damage output by alot to keep a mobs attention on your felguard. Intercept ensures he reaches the target quickly and lets you start casting your spells a little sooner.

Basic killing
Send in Felguard and proceed to apply your dots and nukes. If you have alot of spell damage, you can even kill with nukes alone, using Improved Drain Soul to keep from running out of mana too fast. Lacking fancy abilities, there isn't a plethora of different ways to approach a fight with a felguard. But what he does, he does quite fine. Simple, but very effective.