I work in a theatre, and I see bad etiquette daily (most people, of course, are polite). Here's some rules that I think are important in live theatre.
1) Please arrive on time. If you arrive late, you may not be able to enter the theatre until intermission.
2) If you are unhappy with any aspect of the production, please do not become angry at house staff, who have no control over artistic decisions. Write a comment card, or, if the theatre does not have them, write a letter to the artistic director. In either case, please be civil about it.
3) Be sure to ask about the theatre's policy on food inside the house. If food is not permitted, that means that you may not eat. At all. Just because your sandwich remains in your bag while you pick off little bits of it does not mean you are not eating. If you have a scratchy throat and think you may need cough drops, please unwrap them before the show begins. Theatres usually have excellent acoustics, which means that the entire theatre, including the actors, can hear you rustling your cough drop wrapper. If eating is allowed, do not rustle food wrappers.
4) Please ask before taking pictures in the theatre. In many theatres, the sets are copyrighted, so you cannot take pictures of them. Equally, do not record the show on your cell phone camera or any other device. It may be illegal and is definitely rude, as the light will annoy other patrons.
5) Actually, just turn off all electronic devices. No one wants to be interrupted by your cell phone/pager/blackberry going off. It is very disrespectful to the actors and annoying to other patrons.
6) Remember that this is a live performance, and in all likelihood the actors are aware of what's going on in the house. Therefore, do not talk, make noise with food wrappings/rain jackets/electronic devices or get up and leave the theatre unless absolutely necessary.
7) If you would like to expose your child to theatre (which I am most definitely in favor of) make sure that the show is both appropriate and enjoyable for your child's age and emotional maturity level. PLEASE do your research before you come. Also be sure that your child is capable of sitting quietly through a show.
8 ) If you have any special needs (blindness, hard of hearing, a wheelchair, a walker, a medical condition that means you will almost certainly have to leave the theatre at some point during the show) please notify audience services, preferably when you buy your ticket. Accommodations can almost always be made (a Braille playbill, an audio-description of the action onstage, a seat arranged to comfortably accommodate a service dog, an infrared hearing device, someone to help you to your seat, a seat on the aisle near an exit) but it is much easier if house staff knows in advance and can prepare.