Tip 1: How to accept an invitation
In order to get into a Firelands raid, the first step is always going to be the raid invite. If you accept it, then you're already doing it wrong. At any given opportunity, you should attempt to decline raid invites and ask that a replacement be found for you instead; bonus points if you suggest that the replacement in question not be a shadow priest.
Let's just face the facts, people: You're mages. Your existence isn't really needed beyond the moderate amount of convenience that you provide for people. You give us food, you teleport us places, you wash our cars and talk to annoying telemarketers; you don't, however, actually get to raid with us. It'd be like inviting the help out to a five-star restaurant with you. It simply isn't done.
Stick to the things you're good at. Take your invite, drop a Refreshment Table, then bow out. Oh, and port my alt from Silvermoon to Org, thanks.
Tip 2: Don't allow yourself to become a burden
If, for some unknown reason, a more suitable replacement can't be found for you and you must raid with a group, then make sure that you're at least going to do it right. Ask your vastly more knowledgeable warlock or druid superiors to explain the encounter mechanics to you in the bite-sized words that you'll be able to understand. I know that reading is a challenge that most mages haven't quite overcome yet, so request that they use a VoIP program such as Ventrilo or Mumble when explaining things to you.
Once you know what to do, actually do it. I know that it can be difficult to following the basic instructions of Don't stand in the fire again!, but you really need to stop dragging the raid down. They're already setting themselves back enough by taking you along with them as it is; don't make matters worse for them by ruining it. Further, make sure that you ask for clarification on any instructions that you don't understand. Raid bosses can be tricky for a mage's addled mind; you might need things repeated, possibly multiple times.
Tip 3: Gaining loot
Any raid that takes you in is being generous enough with their time, don't continually push your luck by asking that they actually give you gear as well. Other classes are going to get more use of out those items than you would as a mage, so it's really just in everyone's best interests that you pass off any loot to others before taking any for yourself. In fact, you may always pass a few items over to be disenchanted as well. Enchanting isn't cheap and requires a large number of Maelstrom Crystals. You wouldn't want for a kind guild mate to go without an enchant just because you had to be greedy and ask for the item, would you?
Tip 4: Fulfill your role
Always remember that the real reason anyone brings a mage along for the ride within a raid is to make everyone else in that raid feel better about themselves. No one likes to be the foppish tagalong who really doesn't belong with the rest of the cool kids but is brought along anyway because the others need to save face or look like they care about those less fortunate. You are that person.
Sure, you might not be the best. You might often be passed over for the more complicated aspects of an encounter because they require abilities that you simply don't have. But no one can take away from you the fact that you are the proverbial comic relief. Merely existing in the raid already makes everyone else feel 20 times better about themselves. You're a superstar, champ, and don't let anyone else tell you differently.
Tip 5: Know humility
Never let your position within a raid go to your head. While mages certainly don't suffer from the same alluring ego draw that most other classes are subjected to, the mere fact that you are hanging with the popular kids could just as easily cause you to fall into temptation. It might be easy to let go of yourself and give in to the draw, to lord your elevated status above other, less important mages, but humility is the sign of an inherently good person. When you really don't have much else, you kind of want to be a good person.
Remember, your position is a gift; there are plenty of other mages who would gladly lick the raid's metaphorical bootstraps in order to fill in on your slot. If you don't want this to happen, then make sure you do what every good mage does, and show your gracious appreciation for being allowed to join in on the reindeer games.
Every week, Arcane Brilliance teleports you inside the wonderful world of mages and then hurls a Fireball in your face. Start off with our Cataclysm 101 guide for new mages, then find out which spec is best for raiding, get advice from the poor mage's guide to enchants, and learn how to keep yourself alive.