Grab your addons
There are whole columns devoted to picking out good addons, and I'll defer to Mr. McCurley's epic knowledge on which are best. But as ColbyWolf and Frostymage point out, addons are a very important part of raiding. I'd probably recommend checking in with your raid leaders to figure out what packages to use. Conforming to your group's most common addons will probably increase your likelihood of success; the goal here is to be on the same page.
In a general sense, though, you'll want a boss mod. DBM is my preferred flavor, but check with your raid first. I know a lot of people swear by threat meters, but I tend to find the built-in threat notifications sufficient. (Also, death is a good way to know you pulled aggro.)
Get the scoop on healers
While you should probably know who everyone is before you dive into a raid, as I mentioned before, it's awfully helpful to know who is doing the healing. This is because you need to be careful not to run out of their range, and also because you should try and keep an eye on them. In case of catastrophe, most raids would prefer to lose a DPSer than a healer. Like Cheb says, knowing your healers might give you a heads up on an incoming wipe.
Remember your kit
Take the time to research your flasks, potions and other consumables, and as tkc cautions you, have them on hand. This can be especially important if you're tackling progression content. While a few attack power probably won't make a night-and-day difference for each person, when you get 25 of your closest friends together, that effect stacks nicely.
This also includes stocking up on your reagents. You'll go through more than you think. I always keep enough on me to rebuff 100 times. I've never heard of people wiping 100 times in a single night, but I've never run out of reagents on my own, either.
Be on time
No, seriously. Be on time. Look, I generally try and avoid being all Fanboy Nerdrage Guy, and I try to be understanding and everything. And frankly, I don't buy the "raiding is serious business" stuff, and Sylvanas knows when I don't feel like raiding, I drag my feet.
But if you tell 9 or 24 other people you'll be there, you need to be there. Making them stand on the landing pad waiting for you is a bum move and not cool. And "they know I have a rough schedule" is not an excuse. They are real people, too, who could be doing other things. Nothing gets my goat like wasting my time, and I am not alone in that.
Ready Check shares all the strategies and inside information you need to take your raiding to the next level. For more healer-centric advice, visit Raid Rx, and be sure to look up our strategy guides to Icecrown Citadel and Halion/the Ruby Sanctum.