[edit] Steps

1. Show up on time. Note that if you arrive late, you may be out of luck to getting to your assigned seat for the first act. If an emergency is what prevents you from arriving on time, the ushers will try their hardest to seat you in an empty seat in the back. You should be able to get to your assigned seat by the second act.^[1] And, beware: some actors have been known to point out those who arrive late. It's not a nice experience to have your first few words with a favourite actor to be explaining why you were late.
2. Don't take pictures in the theatre. While it is completely acceptable to take pictures outside of the play, as soon as you enter, turn it off. The plays are copyrighted, and so distributing images of such is considered copyright infringement. Keep the camera in your bag.
3. Turn all beepers off. People will be escorted out of the theatre if their cellphone, pager, or beeper goes off. It is considered distracting to the people around you, and the actors performing. There will most likely be a reminder at the beginning of the play to turn everything off, so make sure you adhere to it. This applies to texting too. Even when your phone is set to silent, the light from it can be distracting to others around you. Shut off your phone until the intermission or end of the show.
4. Leave your chewing gum behind. Loud smacking and popping of gum is very distracting so park your gum for later.
5. Welcome to your seat for the next three hours.
   Sit quietly and don't disturb others. No, it is not necessary to switch positions every three seconds. And the people around you don't want to hear about your hour long high school tales. They came for a reason: to watch the show. Respect it, and don't annoy them. This could result it being thrown out of the theatre. You are allowed to eat, but obnoxiously crunching chips and crinkling the plastic bag will not be accepted. Try your hardest not to irk others. If this means taking off your top hat for the view of others, do it. Remember that if you lean closer to your friend next to you, you're risking blocking the view of the people behind you. Leaning forward or holding a child also prevents other from seeing the show.^[2] And the only time you can talk or whisper to someone is during intermission. If you are hard of hearing, ask the usher for a special hearing device for the play instead of asking the people around you constantly what the actor just said. Also, if you have a tendency to sing along with the actors on stage, don't. Most people came to see the play, not you. And remember -- the actors are live. They can hear you too.
6. Cute, but better left at home.
   Leave the kids with a babysitter. Can you imagine: you're deeply in to the play at hand, and suddenly, a shrieking baby interrupts the entire performance. Would you appreciate it? Children aged infant to twelve often have difficulty sitting still for long periods of time, and will most likely not grasp the story line of the play anyway. For this reason, they are better off kept at home. If you decide to bring your kids, (i.e., because babysitter wasn't available), keep them under control. Many toddlers have gone unnoticed while thumping the seat of the person in front of you. So for the sake of the others in the theatre, watch your kids.
7. Don't leave until the show is over. The show isn't over until the actors have done curtain calls. If you leave earlier, it's considered extremely rude to them because they've worked so hard to entertain you. Waiting a few minutes to race out of the theatre is a small expense to pay to prevent being rude.^[3]
8. Wear proper clothing. This isn't exactly necessary, but is preferred. A while ago, Broadway came with the necessities of highly-formal clothing. Although it is not needed to wear a full blown evening gown and a tuxedo, don't wear a belly shirt and a mini skirt with fishnets and expect to be let in.

Dressing for Men

1. Wear khakis or other appropriate pants. Don't wear shorts or cargo pants. Your trip to Broadway is not the day to show off your new fishing pants. Belts are recommended.
   
   
2. Wear a polo. A nice collared shirt is a great finishing touch. Don't wear a shirt with writing on it. There is a pretty wide range of acceptable items for men when it comes to shirts, so you shouldn't have a problem choosing.
   
   
3. Put on your 'special occasion' shoes. Now is not the time to wear those baseball cleats. It's not necessary to wear very formal footwear. If you don't have nice shoes, then wear your cleanest pair of sneakers.

Dressing for Women

1. Don't wear an evening gown. You can if you'd like, as it will always be accepted in the theatre. But you'd be much more comfortable wearing a skirt and a nice top. You can wear a mini skirt, but if it shows too much skin, it may not be allowed, and you'll have to change. Follow the finger rule. While wearing a skirt, reach your hands down. Your fingers should reach the edge of the skirt. If your fingertips go way past it, the skirt's too short.
   
   
2. Wear a collared shirt or a blouse. The shirt should not reveal too much skin, and should not be advertising cleavage. Consider seeing a Broadway show a family event, not a time to pick up men.
   
   
3. Slip on your good shoes. High heels are preferred, but things like ballet flats are great choices as well.
4. Go easy on the perfume. People didn't come to smell your new aroma, they came to watch the show. A little spritz on each wrist is good enough: you don't have to drown yourself in perfume.^[4]

[edit] Tips

• Dress appropriately.
• Turn off all cell phones, pagers, and anything else that may beep.
• Note that casual clothing is perfectly acceptable for casual, modern shows like RENT or Avenue Q. Anything that you would see the average person wearing on the sidewalk is fine, so don't go too skimpy, but the miniskirts and fishnets will raise fewer eyebrows at these shows.

[edit] Warnings

• Improper etiquette may result in you being forced to leave.
• Don't take pictures or video. Don't take audio clips of the show either.
• Don't distract those around you.