Take care of everything you will need for the raid prior to the start time.
Need to regem/reforge/respec? Take care of it well before raid invites go out. Need food, flasks, or to repair? Again, take care of this before invites go out. Sure, we may all forget to do these things from time to time, but if you habitually have to hearth out of a raid or pop down a guild bank because you forgot something again, your group isn't going to want to raid with you much. When you /afk or feel the need to run off to the reforger every single time you get a new toy, you're wasting the precious raid time of 9 or 24 other people. Time is precious dear reader, and that 15 minutes it took you to gem/enchant/reforge/do whatever to your brand new piece of whatever is 15 minutes that your raid members will never get back. You could have downed another boss in that time!
Be on time.
I was taught the saying in high school: To be early is to be on time, to be on time is to be late, to be late is to be forgotten. If you log on early you give yourself ample time to take care of everything mentioned in the prior point, fly your happy butt to the raid entrance, and this little bit of extra prep time will probably give you ability to get that glass of water or to go tinkle really quickly if you hadn't done so already. Everyone being on time makes for a potential earlier pull, an earlier pull means more things die, more things dying means more potential shinies for everyone. More chances at loot and boss kills make people happy; waiting for Jimmy, Timmy, and Sal to log on doesn't. Savvy?
Know the fights before hand.
Read a strategy guide, watch a video (I hear Tankspot and Learn2Raid have some fantastic ones), do something to get a general feel for a fight before you step into a raid. Do you need to be able to recite the dungeon journal and know what every person in the raid must do in their given role? Certainly not, but you should be at least aware of what is expected of you.
If you don't know, ask.
For a good majority of the raiding community a strategy video, written guide, or the dungeon journal simply isn't enough.While they may understand the basic concepts of an encounter, not everything may "click" for them until they get in there and get their hands dirty themselves. This fact is completely acceptable; we're not machines after all and everyone's learning process is a bit different. However, if by the time your group is a few pulls in and there is something that you or your group as a whole is still not grasping, ask. Ask what you could be doing wrong personally (assuming it's you who is miffing something up), or ask how the group could possibly improve. If you're uncomfortable with asking the group as a whole, whisper either the raid leader, or (if it's a personal struggle on a certain mechanic) ask another person who also happens to be in a similar role as you who seems to be doing well what the skinny is. While you may learn something on your own eventually, asking a simple question may save you many future blunders and repair bills.
Staying positive is by far one of the most difficult things to stick with. Positivity can go a long way in a raid that is either learning for the first time or simply having a rough night. With a short bio break, some positive wording, and a bit of mental regrouping, a raid can go from wiping like crazy to rocking some faces off. Randomly toddling off without permission from the raid leader because you've had it never helps the situation at hand; this includes random afks, leaving the raid unexpectedly, and drama logging (aka so called "power outages" that never actually happen). While yes, bad nights suck, pulling the drama queen out of the closet doesn't help the raid and it certainly doesn't help the people in the raid think any better about you. If you're truly at your whit's end there are many other raids and guilds out there. If you're not anywhere near an actual self boiling point but instead just a bit flustered, try to think positively. Take a swig of your beverage, do a few little stretches, shake out the bad vibes the best you can and get yourself back to killing those pixels. If the boss isn't not downed right then, it can/will be brought down some other day.
Preparation, timeliness, teamwork, and positivity; they make a ragtag team of people into a raid group. Together they slay all sorts of bad beasties, separated they cause a havoc induced headache haven. Take care of your raid and maybe, just maybe your raid will take care of you as well in return.