Whether you want to participate in Player versus Player combat constantly is a choice you more or less made when selecting your server. Playing on a PvP server leaves you open to attack, and it can be annoying to have your questing interfered with by unwanted PvP.
On regular servers this is not the case, and one faction cannot attack the other without question. However, you can still choose to participate in PvP by joining a Battleground, by joining or creating an Arena team at level 70, or by joining the world PvP in Outland.. Warlocks are pretty good at PvP, but are fairly difficult to play. Thee are so many options, so many choices that have to be made in an instant... The smallest mistake can mean defeat.
Some general tips:
If you’re fighting a Priest or another Warlock, Shadow Ward can help to absorb damage, and to prevent spell interruptions through DoT damage.
Fear and Seduce are on the same diminishing returns timer. Using either Fear, Seduce or a combination of the two more then once will make their duration shorter. 50% shorter on the second cast, 75% on the third cast after which the target is immune to both for 15 seconds.
Collect the honor points required to buy an Insignia of the Horde/Alliance, and you'll have a means to break free of crowd control once.
Get a Minor Speed enchantment on your boots. It will allow you to run slightly faster than those without it.
Consider using a Spellstone. Yes, you have to give up your wand for it, but the ability to get rid of a slew of dots with a single click is quite valuable.
Fighting The Classes
To be the most effective in PvP, you will adapt yourself to each class, and use much different tactics for fighting a hunter than you would say, a priest. Note that the below tactics are assuming a 1 v 1 fight, without interference from other parties. While this rarely happens in mass PvP or Battlegrounds, you can still use some of the principles listed below for each class, and maybe learn a little more about the classes you fight as well.

Druids
Druids are a versatile class, and can be fairly difficult to deal with.
First off, Druids are weak against fear. There is nothing much they can do beside their PvP trinket. Dotting a Druid up and fearing him away works perfectly, and you can ruin his spellcast when fear breaks with a Felhunter.
Moonfire's DoT effect can be devoured by a Felhunter, but in the case of it being spammed, he can’t keep up. Devouring their buffs and regeneration effects is also beneficial.
Feral Druids are hard hitters and can be tough buggers when they get the jump on you. And they usually do, being stealthed in cat form.
When a Druid shifts into Dire Bear form, expect to get charged. With the proper talent, they can charge, similar to the warrior’s charge. It will interrupt a spellcast for 4 seconds, and stops you from moving for the same period. Note that there is a 8-25 range restriction on the ability, so you can ruin their charge by being too close or too far away from them to use it. If they are able to get some rage, they will be able to use a heal over time in bear form. Use fear and either start nuking or DoT him up.
Worthy of note when fighting Druids in Cat form; Their Shred and Ravage abilies require the Druid to be behind you. Channeling Drain Life will make you face the Druid for the duration, denying them the use of the ability.
If you get a drui'ds hitpoints low, the Druid will be forced to shift back into their original form, and heal themselves. Its difficult to prevent their healing, but you can use deathcoil the second you see them shift to caster form. This will hopefully have denied them their Heal / Regen spell as well as buying you time to finish the job or follow through with a fear.
Note that you can devour their regen spells with the Felhunter.
For all druid builds goes; Shifting in and out of their various forms costs them a crapload of mana. If you curse a druid in any of their animal forms, it will cost them two shifts to remove it. Most will not bother removing it, as it will cost them a heck of a lot more time and mana than it did for you to apply the curse.
One of the most annoying Druid abilities is Cyclone. The spell works much like a short version of Banish, it makes you invulnerable and unable to take any action for 6 seconds.

It takes them 1.5 seconds to cast, and it's affected by diminishing returns. The second cast will only last 3 seconds. It's still a pain in the ass though.
Hunters
Your best defense on Hunters is to take the fight to them. As you may know, bows and guns have a minimum 8 yard range, unlike wands. If you get too close, they can't use them and have to take action to get away from you. An option is to charge at them, keeping the following in mind; A hunter will usually have a trap up, and they will try to lure you into it. You can break out of a trap with the PvP trinket, but you can also guess the spot the trap is in and move around it. It's usually directly in front of the hunter.
Provided the hunter is at range when the fight begins, they will typically start off with Concussive Shot and Aimed Shot. Concussive will slow you to 50% run speed for 4 seconds. If they have Improved Concussive shot, that will be accompanied by a 20% chance to be stunned for 3 seconds. Concussive shot can be taken care of by a Felhunter or the pvp trinket. Waiting it out makes you prone to further attacks, which can be lethal.
The most annoying specced hunter you can find yourself up against is the Beastmaster. When they pop their little talent, they and their pet become immune to all crowd control for 18 seconds.
There are two ways to screw them out of their talent, more or less. One is to fear them before they can use it. However, even if you open with deathcoil, it's a pain to get fear off in time while his pet is interrupting you. The PvP set gloves help with this though, if you have em. The other way is not denying them the use of their spell, but to simply make it less useful to them by getting out of their line of sight as much as possible. Their pet will still be beating on you, but not taking damage from the hunter is worth that discomfort. You can consider popping in and out of LoS to land instant dots on the hunter. If you have Siphon Life, put it on both the hunter and the pet to alleviate some of the pain.
Another painful ability hunters have is Aimed shot, which does a crapton of damage when they get it off. It takes 3.5 seconds to cast, which is an eternity for you to make sure you are either out of range (too close or too far) or interrupt it.

Its worthwhile keeping mind of the fact that hunters cannot dispel anything period outside of the pvp trinket. You can toss dot's on them and watch them tick away while you're out of their line of sight.
Using seduce rather than fear works fine as well. You could send your suc in to seduce the hunter before they activate Bestial Wrath. While the hunter is seduced, kill off their pet.
I would not normally recommend killing any pet, but it can be worth it in the above scenario.
CoEx is an asset against the hunter. They rely greatly on being able to stay in shooting distance of you, and if you deny them the ability to catch up (or get away fast enough) it will seriously hamper their damage.
Mages
Mages can be a problem, but once you have some experience fighting them, they tend to become a lot easier to deal with. Two important things to keep in mind are:
-Many of the mages instant spells must be cast from up close, so maintaining distance is important if you wish to do your best to avoid that. They often use their blink spell to try and get close enough to you for their instants.
-Mages can remove curses.
Casting most any curse is a waste of mana if the mage isn’t feared. However, using Curse of Tongues can still be worthwhile, especially considering its low manacost. Removing the curse doesnt take much time either, but stretching out a fight is usually in the Warlock's favor.
A mage will typically attempt to start a fight off with either poly or their instants, as to let the cooldown on those reset as fast as possible.
Should they start off with sheep, either spell lock or seduce them if you have no other way out of it. (Felhunter or Succubus is best used when fighting a mage) If they break out of seduce and sheep you again, either use the PvP trinket to break out of it, or simply seduce again. If you are undead, you can use WotF as you see them cast poly.
A Mage will counterspell you when they see you casting, which prevents you from casting spells in one school for 10 seconds, similar to the Felhunters’ spell lock. If they have the Improved Counterspell talent, you will also be completely silenced for 4 seconds. If you have a Felhunter out, you can devour this effect. You can sometimes get a mage to blow their silence by casting fire spells. If they shut down the fire school you aren't enarly as hindered as with the shadow school.
Counterspell is on a 30 second cooldown.

Start off by laying dots on the mage. Corruption, Immo, Siphon, UA if you have them. Usually, you will be counterspelled as soon as you cast Immo. If not, follow up with Searing Pains or incinerate until you do.
Fearing a mage after their Counterspell hits you will often win you the fight. DoTs are ticking away while they have no way to prevent your damage, outside of possibly Ice Block.
Undead mages are a little more annoying in that they potentially have two ways to break out of fear. (if they have the pvp trinket equipped) Fearing them three times in a row isn't really worthwhile considering the damage you take during the time it takes to get those fears off. If they choose to get up close and personal to let their instants fly, keep your finger on the healthstone.
A Soul Link Warlock will have a much easier time dealing with mages then any other build. Switching pets instantly can also be quite a nasty surprise for a non suspecting mage.
Frost Mages often use Ice Block when they’re in trouble, which buys them 10