There's a discussion thread going on in alt.games.warcraft about Warlock specs and how to use them. I found this one post particularly interesting and informative for Warlocks who want to play the Affliction tree. Note, I don't have a warlock so I didn't write this and have no real opinion on it, other than thinking it's interesting and wanting to share:
Q: What is the best method of grinding? What pet should I use for grinding? What is "drain tanking?
A: "Drain tanking" is generally the most efficient method of grinding as a Warlock after 40. Basically, you're using Fel Concentration to help you use Drain Life on the mob until it dies, while your Succubus is beating on it and DoT's are ticking on it.
Here is the general "drain tank" strategy:
Locate mob. Succubus should be out, with no auto-casts activated.
Open up with Immolate/Shadow Bolt.
Stack CoA and Corruption
Mob is in your face. Send succubus to attack.
Drain Life. Drain Life. Drain Life. Shadow Bolt if Nightfall procs.
Mob is dead. Dark Pact to regain mana. Succubus should have no auto-casts activated so that all of her mana can be used for Dark Pact and the emergency Seduce.This strategy, of course, is made to be used with the Dark Pact build.
Some Warlocks incorporate Siphon Life into their Drain Tanking strategy. The common consensus is that Siphon Life is only mana efficient if it lasts its entire duration. That means it can be added if you're fighting a tough mob.
As you obtain more and more damage gear, you may want to switch to the Imp with Fire Bolt turned off. You'll have practically no pet DPS (a little bit from Fire Shield), but the Imp's mana regeneration is just ridiculous at later levels - just remember that you won't have Seduce to help deal with ganks. Experiment and find what is better for you.