Locks are an easy target in BG's. I usually don't do BG's until I am at least 2 lvl's below the bracket limit. (Ex: I wait till I am at least 37-39 before entering BG, then I power lvl till I am 47-49) That way I won't be over powered by upper lvl rogues, hunters, etc...
(I am a 49 lock, affliction build)
The other day, to my surprise, I managed to capture 2 flags in WSG for the Horde. I always thought that since locks wear cloth and everything, I would be dead before I even got out. I think I had a Warrior stunning and a Druid healing me on the way though. And anybody they couldn't handle, I got w/ my deathcoil and kept running.
On the offensive though, I try to keep my distance and send in my Succubus. Setting Seduction on Auto-Cast is pretty helpful because it helps me when I'm being 2 vs 1. In the case when I am being double teamed, I use Seduction on 1 guy, fear/deathcoil the other guy, Health Stone/Potion myself and RUN and call for help, LOL. By the way I cast Siphon Life on everyone in sight. so my health will stay full, that way I can Life Tap more often. I also carry a few mana potions w/ me as well.
If, however, my team's offensive sucks...I get bored and go on the defensive. I usually go up on the roof since my shadow spells can reach down into the flag room. I have my succubus stay near the entrance of the roof on defensive, so if anyone decides to sneak up on me, the succubus can sneak up on them and surprise attack (hopefully she can crit), I fear them, DOT, siphon life, drain life, and get a soul stone. Then I cannabalize (I am undead also)
If someone manages to get to the flag, I cast siphon life, DOT then up, jump down and death coil them, and rescue the flag. There usually should be a hunter or priest with you on defend too.