I actually inadvertantly "twinked" my lock at 39 for bg's by a series of experiments. I'm an affliction lock so I'm always on the look out for cheap + shadow dmg gear in the ah, and that is key to twinking.
As I say, I picked up a lot of + shadow damage stuff, in addition to a lot of + stam and + int gear along they way. I had also been running a lot of wsg and ab games to build up the honor points and tokens for the upgrades of gear for the next round of leveling. Before I began my experiements, I asked my guildies what the best course of action was for a good lock in bg's (being fairly new to the game...more of a d2 mf expert). All of the locks in my guild said that the gear with + int and + stam was the way to go but, being me and scientifically minded, I had to check out both their suggestions and my dmg gear for my own peace of mind. I'm glad I did...their advice was completely wrong. Affliction locks perform best with dmg gear on. To be totally honest, despite all the lack of stam, you still die about the same amount of times. Key is playing sensibly in all bg situations...that means staying as far away from melee classes as possible.
Here's a list of my gear at 39:
Shadow Hood - +11 int, +7 shadow dmg, lvl 29
Pendant of Myzreal - +4 stam, +6 spirit, lvl 30
Elder's Mantle of Shadow Wrath - +13 shadow dmg, lvl 28
Black Mageweave Gloves - +15 dmg healing, lvl 38
Shadoweave pants - +6 stamina, +21 shadow spell dmg, lvl 37
Regal Cuffs of Shadow Wrath - + 15 shadow dmg, lvl 36
Advisor's Ring - +9 spell dmg/healing, 3 mana over 5 sec, lvl 38
Wicked Moonstone Ring - +5 int, +6 shadow dmg, lvl 27
Defilers Boots (cloth) - 103 armor, +10 stam, +3 int, run speed improved, +8 spell dmg/healing, lvl 38
Defilers cloth Girdle - +4 stam, +4 int, +14 spell dmg/healing, lvl 38
Advisor's Gnarled Staff - +14 stam, +11 int, regenerates 6 mana per 5 sec, lvl 38
Rune of Perfection - lvl 20, +4 stam, +10 spell penetration
Insignia of the Horde - Removes movement imapring effects (believe the alliance does the same)
Shadoweave robe - +7 spirit, +21 shadow dmg, lvl 37
Battle Healer's Cloak - +8 stam, +5 spirit, +18 healing spells, lvl 38
All the additional damage really helped a lot. Not only did I consistantly come within the top 3 of damage done, but also usually within the top 2 of honorable kills. Deaths are easily avoided...don't be a lock that runs in, dot's everyone and then waits to die.
Additionally, use a felhunter in bg's. Not only can they do well with the melee characters, they are great for dispelling magic from others who follow magic disciplines. One thing is that characters loose a lot of their magical protections once a felhunter has removed their effects. For lvl 39 twinking, all other pets are useless...unless you want to be one of those warlocks who doesn't have a clue how to play and keeps fearing and putting a succubus on people (they are sitting ducks). Having the Insignia of the Horde and, as undead, having will of the forsaken, it is easy to get out of their silly little games, dot them and fear them to death.
Most importantly, learn your best spells. I never see any other warlocks using banish to get rid of pets...that is very important when playing against another lock. As they never use this, and I do, I can get rid of their pet set my fel on the lock and remove their magic effects. Also remember to use amplify curse when you can. CoA and Siphon Life are best starters as you can do those on the move.
Remember also that a good lock is one that helps the group. I find it best to kite enemies with my group, COA and SL all, as that helps others get them down faster. One other tip, if you are going to flag run in wsg is to send your pet into a group of enemies while you run in the other direction. That way, players go mental looking for a lock that's not there and are distracted by a pet. With a build like I had, you can play effectively without a pet for quite a while.
I'm looking forward to going in at 49, with new spells and equipment I am already building up.
One other point is to make alts with different professions. Having a tailor, enchanter, jewelcrafter and alch really help and cut down ah costs tonnes.
Hope this helps some. Good luck!