One important thing that a lot of DPS neglect is good raid frames. Even though you're not healing, raid frames serve two purposes: 1) to tell you when people are within range, and b) when they have a dispellable debuff. Of course, keep your Power Word: Shield keybound and be ready to use it not only on yourself, but on allies.
I recommend an addon like Bartender or Dominos to replace the default bars, which are pretty poorly designed.
A lot of people swear by ForteXorcist as a DoT timer - I myself have never bothered to use it, since I rarely go Shadow, but I've heard very good things.
DBM (Deadly Boss Mods) is not only standard issue for raiding, but also very useful for BG's to show when certain points are going to be captured.
One alternative is just grabbing a premade UI from Wowinterface or Curse or someplace.