Well, after being in many interesting situations with one of my groups, I've more or less developed the following ideas:
1) Don't get upset at death (I once went through about 10 characters in about 2 sessions

)
2) Go with the rules your DM uses, whether they're right or not. Sometimes they can even make more sense than the ones in the book (an idea I still need to work on a little, though I am getting better). It's just not worth it to have a fellow player or DM angry with you.
3) Get to know the people you game with, even if you only see them when you play.
4) Never attack another member of the party (PC or NPC) without reasons.
Real reasons. "Well...I am evil" isn't a reason, nor is "He's insane." PCs like that have always either repented, died, or got everybody killed in the games I've played.
5) Never do anything weird or unnecessary to PCs (a lesson I learned after another PC did some rather "questionable" things to my bard, one of my PCs that only lasted half a session).
6) Bad stuff happens. Find the irony and laugh when it does, then fix it.
7) Don't steal from others. Divide the gold equally among the party. Usually, it can easily be decided which item would be best for which character (if not, Rock-Paper-Scissors is just as good

).
I didn't include "everyone should have fun" in the list because I didn't really learn that from anyone, it's just something I don't think one should really have to concentrate on if your in a good group, it should just happen.