Most of this will apply to classical music venues:
1. Get there at least a little early so you can be settled by the stated start time.
2. If you need to leave your seat for any reason, remember that the venue is unlikely to allow you to resume your seat until the current piece is between movements or has ended. Opera houses will not allow you to resume your seat until intermission.
3. No food or drink is permitted, especially any that will make any noise. This is not limited to potato chips.
4. If you have chocolate or hard candy, remember that crinkling paper will annoy other concert goers. Either unwrap the item between musical pieces or under your coat on your lap.
5. No talking, for obvious reasons.
6. No leaning your heads together. This obstructs the view of the person behind you.
7. If you have a cold, bring cough drops and follow rule regarding wrappers. If you have a very runny nose, bring enough tissues to take care of the problem without having to blow. If it's too bad to follow this rule, do not attend.
8. Shut off all portable communication devices.
9. If you need to leave a row, face away from the others in your row and say "Excuse me" as you pass. Facing toward them (if there is the usual lack of leg room) will cause you to fall into them or step on their feet.
10. If you are one being passed, accept the apology graciously. If the seat flips up, stand if you can to make it easier for them to pass.
11. Dress decently for the venue. Classical music events are not cheap and you will not want to look like a slob in a venue where people are often dressed to the nines.
12. Follow the rules regarding applause. These may vary by venue or composer.