Cell phones are the worst, especially in a small theater. Turn it off. Or don't even bring it in the building, especially with that ridiculous ring tone that goes on and on. But also silence beeping wrist watches, hearing aids, anything that could and probably will make a distracting noise at an important part of the show.
Save the discussion group for intermission and the cocktail table afterward. Side chatter, especially whispers that are loud enough to be heard a few seats away, are almost as bad as the cell phones. Don't worry. You'll have plenty of time to talk about whatever happens, including during the drive home and any idle moment during the days that follow. If you are afraid you might forget the point, silently jot down a quick note on your program. Never, never, tap your friend and tip off that something really important in the plot is going to happen in just a few seconds.
Don't hog the armrests. Two people share every one, except on the aisles, so think about that before you claim the turf. The armrest is meant to be a divider, not a retaining wall.
Eat at home. Unwrapping candies or snacks makes that crinkling, crinkling sound that can't be blocked out. Even worse, some people compound the grazing with burping or other bodily sounds. When you yawn in a theater, keep the noise contained to the air you are sucking in. Don't make the waaaaahhhhh sound, too. If you are sleepy, stay home and skip the show. No one wants to hear you snoring.
Don't come in late. Or leave early. Both are distracting. The curtain call, by the way, is one of the most joyful parts of the performance for the actors, and they usually have earned the applause. That's part of what keeps the performers energized and engaged in their craft. So it's rude to hustle out of the theater at that point in the show to try to beat traffic. If you don't want to get caught up in the crowd, just wait in your seat for 15 or 20 minutes after the curtain closes, and the theater will be clear.