This is specific for the Boise, Idaho comedy scene, but some rules are universal.
First off, what is an open-micer? An open-micer is someone in the comedy equivalent of little league, someone who either is too new or not good enough to perform in anything other than at an open-mic. In some cities, like Boise, comedy clubs are infrequent citizens, so open-mics might be the only opportunity that you have to perform at all. There's no shame in it, the funniest stand-up comics in the world have all had to start somewhere, and even experienced comics will come out and play with the rest of us from time to time.
There is no average open-mic. If you're lucky, the local comedy club will run a room a few times a month for you, but you might have to start out at a coffee shop going up on stage between a slam poet and a musical act; the rules, however, are pretty much the same:
#1 - Show up. Show up early and show up often. Most open-mics have sign-up times and lists; many don't put the set order together until right before the show starts but, either way, make sure you are there. Open-mics are like gyms to athletes, and if you don't work out your comedy muscles, don't be surprised when you get all soft; even experienced comics need stage time to resharpen their skills once they've taken some time off. If you show up late, don't be shocked if you don't get on stage. We live in the modern world now and if you can't find some way to communicate that you might be getting off work late, the world is going to blame you for it.
#2 - Be prepared. It would be nice to be talented and awesome enough to be hilarious every minute on stage without having to think or do anything about it. If you are already that good, what the hell are you stealing someone else's stage time for? For most us, there is an entire process that goes into writing and performing a joke and, if you don't have that process down yet, you should figure one out. Most open-mics are pretty lax about performers bringing up a setlist to work out some new material, but you're going to be a lot more confident and more funny, if you're 'off book'.
#3 - Be funny. The audience is expecting to laugh. The general rule of thumb with setlists is to close with your best joke and to open with your second best joke. The audience is doing you the favor of letting you work out new material, but make sure to keep a couple of good jokes at the ready just in case you falter, your new bits don't go over well, or the guy in front of you really sucks and they need to be roped back into the show.
#4 - Write more jokes. The biggest enemy is always complacency. During a recent interview, Louis C.K. said that the only way for a comedian to get any better is to throw away all of his old material, otherwise you'll be tempted to fall back on it. I do it, hell, every comic is tempted to fall back on the old material instead of going through all the work of putting some new stuff together. The only way for you to achieve any of your goals as a comedian, even if it's just making the same audience laugh over and over again, is to bring something fresh to the table. The best comics write every single day, either new stuff or fixing the old stuff. Either way, this is one of the best ways to get better.
#5 - Don't be an Asshole. Showing up drunk, being wasted on stage, yelling at the staff, not tipping your waitress; all of these offensives are observed and opinions are made by your peers and the audience. You could be the funniest comic in the world but, if nobody likes you, nobody wants you around. Comedians by nature can be very contrarian and confrontational, but be as charming as possible. If possible, tip the staff extra. You'd be surprised how much the opinion of the staff can change the atmosphere at an open-mic. If you can't afford to tip, you can't afford to drink. Most open-mics have a specified time they expect you to be on stage, don't go over that time. At all. Comedy Clubs and open-mics in Boise usually use a light to let you know when your time is almost up, make sure to look for it. Ignoring the light is the second most common reasons why some comedians disappear after a show and are never heard from again. Every minute you go over your time, I have a new fantasy about where I want to stick an icepick.
#6 - You don't deserve anything. Odds are you're only half as funny as you think that you are. If you're like everybody else on this planet (protip: you are), you grossly overestimate your own abilities and only listen to opinions that validate what you want to believe. You should be thankful that someone has put together a location and audience for you to perform in front of. If you don't like how something is run, you are always free to experience the headache of organizing and managing your own room but, trust me on this one, it's a shitty job. Remember this the next time you want to bitch about how quiet the audience is at this location, or where the room wants the comics to sit while waiting to go on stage. Even headliners in comedy clubs are frequently forced to sit in green rooms during the show and sometimes aren't even allowed to talk to the staff. Always be respectful of the other performers, the audience, and the staff, and especially to the person hosting/managing the show. The biggest headache of my week is working with my fellow comedians, and every time someone complains about their placing in the show or how much time they get that night, I get one inch closer to shutting the room down entirely.
#7 - Be supportive. The audience that comes to an open-mic can be a fickle thing. It can take months to build a regular audience, and you still will have shows where only three people show up. If you are genuinely interested in working on your craft as a stand-up comedian, this is an excellent time to learn how to market yourself. In some places you aren't even allowed on stage until you bring in three people, and no bar is going to keep a room available unless there's people there buying drinks. It is in everyone's best interest to do everything you can to keep that stage available. In one instance, my then girlfriend, a fellow comic, and I spent three hours in the summer heat passing out flyers we made on our own to get less than a handful of people to come to an open-mic at our local comedy club, but at least we had someone to perform in front of that night. If you are a smoker performing at a non-smoking show, but only come inside for your set, you get your own level of hell where midgets spend eternity poking you in the legs with sharp sticks.
#8 - Have a good time. Stand-up comedy is entertainment, it's about escaping your crappy job and watching someone who can make you laugh. YOU ARE SURROUNDED BY FUNNY PEOPLE. If the audience is nervous because the show is in the daytime for some reason and they can see themselves laugh, if the last six comics completely bombed and the MC is drunk and messed up your name, if you are brought on stage immediately after a tear-jerking speech at a benefit show for premature babies (this happened to me), walk up to that microphone and have a good time. Comics who are enjoying themselves are ten times funnier and twice as likely to get laid after the show*. If you suck at every other thing on this list, do this last one. Life is a mess and the good parts are too short, so the next time we're standing next to each other and I'm handing you the microphone after ordering the audience to clap for you, try to remember that for the next several minutes you get to be a comedian, and that's one of the coolest jobs in the world.
* - Results may vary based upon attractiveness of comedian.