While I'm leveling my disc priest/4 locks for PvP, here's my PvE (i.e., questing) strat. I use 2 voids, a dog, and an imp. The only thing the dog does is counter spell when needed. The dog and imp are on passive, and the 2 voids are on defensive. All locks are affliction, with the one with the dog having improved felhunter, and the 2 with voids have points in the demo talent that heals your pet. The one with the imp has improved imp. Then, do like Owl said, run around dotting everything in sight, whether they're quest mobs or not.

My team is sitting at 72 and since they got seed of corruption...holy jeez. Round up as many mobs with the two voids as possible and start throwing 5 seeds on about 3 different mobs and watch the fireworks.

The only thing about not having an actual tank is you're going to have some issues until you get some gear, since they really nerfed pet's tanking (from what I understand, I could be incorrect.)