Warlocks can be a powerful and potent force for PvP, but a little planning and skill separates a bad warlock from a great one.
Warlocks embrace the dark side in World of Warcraft. Warlocks willingly consult with demons and risk their sanity in their efforts to learn to control the powerful demonic forces that a warlock commands. It is not surprising that a warlock would wish to aid his side and further his own goals by seeking out opponents, worthy or otherwise, in player versus player (PvP) combat.
The similarity of the warlock class to the mage class makes it seem that the two classes would have similar PvP play styles, but this is not the case. The addition of the warlock's pet and how his most effective spells damage the enemy call for different strategies.
A few of the warlock pets serve the same purpose in PvP combat that they do in player versus environment (PvE) groups. The imp adds to the stamina of the group and provides a little extra damage. The voidwalker is good for tanking, and the succubus can be used for extra DPS or crowd control Often a warlock pet is not used the same way in a battleground that it might be in an instance of a raid.
However, the pets that function the best for a warlock in pvp are the succubus and the felhunter (and at higher levels, the Felguard which can both take and deal more damage than the voidwalker.) The felhunter has a high damage per second (DPS) output, but its primary use is in stunning casters so that the warlock may cast his dots and the warlock's caster opponent cannot cast spells on the warlock.
The succubus is valued in PvP combat for both its damage and its crowd control abilities. The charm of the succubus does not last long in PvP because the succubus's charm ability is most effectively used to buy the warlock time to cast his most powerful damage over time spells.
The fear spell can be used to cause the succubus to attack an opponent from behind. Unlike PvE combat, fearing an opposing player does not cause an additional amount of opposing players to come back to the fearing party.
A warlock's pet provides additional damage, but the warlock should rely on his brains and spells in PvP combat as much as he does in raids, soloing, or groups. Spells like soulstone, which brings a warlock back into a fight, should be used before going into a battleground or arena, although the soulstone itself should be used when there are no enemies around the warlock.
Lifestones, potions, and band-aids work when a healer cannot heal the warlock or the healer is unavailable. In a pinch, the warlock can try using the damage over time spells to drain life from his opponents, but a combination of all these techniques work well. The best tactic is of course, not to get hit.
While the word count limits prevent this article from being a comprehensive guide to warlock PvP combat, the tips presented should help a player figure out what he needs to do as a warlock and to truly excel at PvP, he should specialize in the affliction branch.
Resources:
World of Warcraft Warlock Talent Tree
World of Warcraft First Aid Guide
World of Warcraft Engineering Guide
Free Warcraft Guides Warlock PvP