I first posted this in my new guilds forum which has a couple of raiders who are for the first time in a progression guild. With Cataclysm bringing a fresh start you may find yourself in this position to, but that's not to say these 3 simple things won't up your game either!
First off RAWR is officially working for Cata, use it to talent, gem, enchant your gear for max dps hps tps ttl etc as well as building a wishlist of gear (you may find some blues are better than raid gear so could possibly pass them for shards! Especially trinkets!). If you've ever heard of min/max-ing this is what you do it with, make sure you edit the buffs to represent what you'll be using in your raids though, otherwise it will spurn out completely wrong stuff. Also to mention is for Tanks, edit what you prioritise most, whether thats TTL (time to live), TPS (threat per second) or Avoidance, if you're OT remember to go for expertise cap over avoidance as you're going to want to be able to overcome the MTs threat when its time to switch