As a melee class, there are a few special rules we need to follow to be the most helpful to the raid. If you follow these (and believe me, they're pretty easy), you should die less and everyone will love you. Except that one guy. But he's a douche...
Don't stand in fire.
I know, I know. We can beat this one to death, but nevertheless it's still one of the most important rules of raiding. You may be tempted to stand in it for "just a second" to get that last hit on a boss before you move, but don't. Your healers will hate you. You might die. And you just look like an idiot. Just. Don't. Do. It.
Wait for the tank. (dpsing before boss is in position)
Tanks have to establish some aggro, and with Cataclysm it takes a little longer than it did back in WotLK. This is especially true if they need to move a boss into position, such as Cho'gall or Atramedes. Dpsing early may cause you to pull aggro and die very early on in the fight, which basically makes you completely worthless. Either you'll do no dps (because you're dead) or they'll need to waste a battle rez on you that should be saved for tanks and healers. It's very annoying to your tank, who has to work harder to gain aggro, and your healers, who will have to use healing cooldowns on you if you happen to live. Just wait for the go ahead from your tank or raid leader, and you will live longer and do more dps in the long run.
Don't break CC.
This is very annoying. If there's any sort of CC near you, you should be using NO AoE spells. Broken CC can easily wipe a raid. The only type of CC you won't break is banish, and even then you should pretend and practice not AoEing.
Look up your class and find out exactly how much hit and expertise rating you need to be capped. You will do more dps, your CC abilities won't miss, and overall it's just better to be capped.
Generally, you want to be behind the boss. Bosses can't block or parry from behind, only dodge, so as long as you're expertise and hit capped, you shouldn't miss on your special attacks. However, on some bosses, dpsing from behind isn't an option, such as the two dragons in Bastion of Twilight. When dpsing Theralion, the usual strategy is to have all melee dps stack on the tank. This means you must dps from the front, and you will be parried and lose some dps. However, staying alive will increase your dps, which wins in the long run.
Melee all have some form of viable interrupt that you'll probably be asked to use at some time or another. Always have this in an easy to reach keybind so you're ready if you need to use it. Missing an interrupt on bosses such as Halfus (Shadow Nova), Omnotron (Aracanotron's Aracane Annihilator) or Cho'gall (the add's shadow thingy) can equal a raid wipe. Know the cooldown of your interrupt(s) and if you need help with them, ask your raid leader to have someone else backing you up.
Work as a team.
It's never good to forget that your raid has to work like a team to down content. Even if it seems like you're carrying the dps meters, remember that you can't kill a boss on your own. Acting high and mighty will only make others mad at you - nobody likes a snob. You don't want to lose your raid spot or the respect of your fellow raiders because you forgot that you're not the only one in the raid.
Blow damage reducing CDs and healing spells if you need to.
This is a big one. A lot of dps will be so focused on their own rotations, positioning, and interrupts that they'll die to a boss's AoE. A big example of this is Chimaeron. Enhancement shamans can be the most helpful to their healers here (read this post to find out how), but other classes such as rogues and death knights can use abilities like recuperate or death strike to alleviate some of the healers' stresses. If you find yourself kind of squishy, level first aid and bring bandages to raids.
Hopefully this will help you stay alive longer in boss fights and boost your dps to boot!