I've been Drain Tanking since my mid-twenties when I started pulling aggro off of my Voidwalker. At first I went with a variation of Drain Hunting, but then I found my Voidwalker to be obsolete once I hit 40. Reason being....
Dark Pact FTW!
Affliction build Warlocks, make sure to pick up Dark Pact as soon as possible. Once you hit 40 you should have Dark Pact, and with that, you will most likely never have downtime. The trick is to drop your Voidwalker and switch back to you Imp. Keep your Imp out of combat (keep him Phase Shifted so he doesn't pull aggro), and use him as a mana battery by using Dark Pact. When your Imp is out of mana, simply Life Tap then Drain Life, and/or bandage up (or Cannibalize if your Undead) if necessary and within a few minutes your Imp should be fully recharged on mana. At first it may seem like you might be dropping in damage, but the amount of damage that your pet does is nothing in comparison to your nearly complete lack of downtime.
Click
here for a sample of what this build looks like at 40, or
here for a sample of a similar build at 70. Keep in mind that there are many other variations of this build, I'm definitely not the first one to come up with this build or the strategy that works well with such a build. There are plenty of other great builds out there, but I've found it that Affliction builds tend to have significantly less downtime (if any downtime whatsoever) in comparison to the other builds -- the lack of downtime completely offsets any loss in damage that such a build might suffer.
Mana Battery Imp Minimum Level: 40
Mandatory Talents: Dark Pact, Fel Concentration, Soul Siphon, and whatever other Affliction points that are necessary to pick up in order to get Dark Pact.
Boosting Talents: Improved Corruption, Improved Drain Soul, Improved Life Tap, Improved Curse of Agony, Amplify Curse, Grim Reach, Nightfall, Empowered Corruption, Siphon Life, Curse of Exhaustion, Shadow Mastery, Contagion, Dark Pact, Improved Howl of Terror, Unstable Affliction, Demonic Embrace, Fel Intellect, Improved Imp, Improved Shadow Bolt, Bane, Shadowburn, Devastation, Ruin, and/or any other talents that complement an Affliction build.
Description: This is a relatively simple variation of the Drain Tanking build, but instead of using a Voidwalker as an off-tank, one switches to an Imp and keeps it out of combat (with Phase Shift up) in order to use him as a mana battery (via Dark Pact). With this Drain Tank variation, a Warlock uses their pet's mana as an extention to their own mana pool and will most likely never have any downtime. Alternatively, one could use other demon pets -- even though Imps seem to regenerate mana the fastest, sometimes its helpful to have a Succubus out (for the use of Seduction), especially on a PvP server where one sometimes has to contend with player aggro.