As player:
-Listen to your DM, don't interrupt him/her, fellow players or the mood.
-If you have a question, make a note of the answer. (Don't ever think you will remember it, the third time you ask it again is a reason to get kicked out)
-Simple is good in whatever way. (Powers, characters, goals, names, etc.)
-Set a goal and a backstory for your character. It doesn't need to be Shakespeare, but a solid short history and reason to go out and adventure gives depth to your character. (Your DM will love it! It will inspire him/her to give you what you want in the way you want it.)
-Respect other players reason to be at the table. Don't bully, harrass or boss them around. If they act in character, treat them as how your character would treat them. (And don't talk about their character in player names: "Robert, you'll just use your gay elven powers to do shit, alright?")
-The DM is not a fun dispencer. (You can provide fun for yourself and others, too. The DM is not a puppet master you have to stare at like a television.)
-The game is not about (just) you. It's about a group of characters in a fantastic world. Each character has his/her own story.
As DM:
-Listen to your players, they can give you better ideas than you can come up with.
-Don't worry too much about details. They will come.
-Players don't always show their emotion or appreciation, but you're doing a better job than you think. (Do you show emotion when watching a good television show?)
-If it sounds great in a story perspective, go with it. (Even if that monster you worked so hard on was instantly killed by a closing door.)
-Give some breathing room. (time to socialize, interact, break times, ups and downs in climax, etc.)
-Be clear about what kind of feel you want to make your campaign. Be clear in general in what grand goal you want to give the adventurers. (They aren't psychic, neither are you.)
-If you think the puzzle/challenge is okay to figure out, it's probably too hard for the players. They need to sift out all the options you didn't thought about.
Have fun.