Afford yourself every possible advantage outside of raiding.
This means:
Full consumables, flasks, speed potions or strength potions, health pots, stack of bandages, applicable buff food.
Best possible enchants and gems for every slot, don't assume what to do consult a spreadsheet to understand why you're doing what someone else told you to do.
Go into raiding looking to improve dramatically on every angle. For me I like to know every single corner of a fight, what damage is dealt when, to who, at what interval etc. Knowing this affords me the flexibility I need to always achieve maximum possible dps. When in doubt, find out what will kill you, and then aim to avoid that. Hope this helps, and hope you're less nervous your next raid. If it makes you feel any better I am applying to a new guild this coming Tuesday, and I have to live up to a lot of hype! No fear though, I am not nervous :P Happy raiding