Be on time. Coming to a performance late is rude for both the performers and audience. Climbing over people to get to your seat after the show has begun is simply not acceptable.
Go to the restroom before the show starts. Once again, you don't want to climb over people while the performance is going on.
When the lights dim, your mouth should close. Absolute silence in the audience is the best way to ensure that everyone can fully enjoy the performance so when the show is about to begin you should not be talking.
Cellphones off. This is an obvious one, but so many people disregard it!
No whispering. We can all hear you no matter how quiet you think you're being.
If your child is making noise, step outside. It is so frustrating to be a performer who is trying to be heard over a crying child or an audience member who is attempting to hear despite a disruption like this.
Don't leave until the final bow is taken. I know you want to beat the crowd and think you're clever, but in actuality you are being rude to the performers who have just worked hard to give you a great experience.
Not so hard, right? These guidelines go for any type of performance and can also be applied to the movie theater. Did I miss anything?