General
Don't touch other people's props. It leads to misplaced, missing and broken props.
Don't give the other actors direction. That's what the director is for.
Always be on time to call, if not earlier. Being late wastes everyone's time, and causes much stress.
When the stage manager makes an announcement (ex. "5 to places") respond clearly, quickly and quietly with a thank you. (ex. "thank you 5"), so the SM knows you've heard. This doesn't apply exclusively to calls of time.
From the beginning of put in until strike is over, the SMs are in charge of everything related to the show. Period. Not the director, not the faculty, the stage management. If they choose to defer to others in certain matters, this should not be construed as their having placed others in a position of authority. Always defer to them unless explicitly told otherwise.
Pay attention to your stage managers when they speak. And then do as they say. They should try to remain courteous, but they may not always have time to deal with you individually or explain themselves.
Don't presume to do the stage manager's job. If you see a problem, inform them. They will deal with it.
Don't eat/smoke/drink in costume.
Never touch anyone else's costumes.
If you create a mess, clean it up. No one on the crew is your janitor.
No talking backstage, and in general remain quiet at all times while you are in the space (green room, dressing room, backstage, etc.). This helps those who need to communicate to do so, and doesn't chance distracting the audience.
Stay involved with the show as it progresses and try to stay in earshot of the action. That way you'll never miss a cue.
The stage-hands are part of the show. Try not to get in their way or cause them problems.
The stage-hands and ASMs are not your mother nor your slave. Unless you know they should do it, don't make them look after your or fetch and carry for you.
Look out for one another, both on and off the set. Injuries suck, and so does missing a cue.
Remember to be courteous to your director both during and after rehearsal and performance. No one likes to be treated with disrespect, and everyone enjoys a pleasant greeting and a kind word.
The actors are part of the show. Don't make their lives any harder if you can avoid it.
Headset
The following are a few guidelines for headset etiquette:
Arrange with your stage manager when you should be on headset, and then act accordingly.
When you get on headset, announce your presence.
When you get off headset, announce your absence.
Always get permission from your stage manager before going off headset.
Acknowledge your warnings and standbys.
Take your cues off the stage manager.
Unless you're the stage manager, put your headset in the mode that transmits only when you press the button to talk, rather than the continuous transmission mode.
Dissuade actors from yelling into the headset microphone.
Do not distract the stage manager from calling the show.
Do not talk during standby.
Respectfully stop talking any time a stage manager requests silence.
Above all, respect your stage manager and each other.