When you go to an open mic, you will rub shoulders with new and old comedians alike. Experienced comics meet brand new comics all the time and will judge them on their behavior, as well as their material.
Most gigs come through networking, and new comics should be wary of breaking the unwritten rules, otherwise they might have trouble getting gigs later on. It should be obvious why: Working comedians do not want to work with people who make their job harder.
If you go to an open mic to perform, remember your goal: make the show better. You won't always succeed but that is essentially the spirit of all the rules to follow.
The rudest thing a comedian can do to another comedian is interrupt their set.
Everyone hates it when audience members are having a conversation during the show: it's rude to the comedian on stage and it distracts other audience members. Comedians are not exempt from this rule. If you are having a conversation, leave the room.
If an audience member walks in and sees an empty room and a bunch of open mic comedians waiting to tell jokes, they will leave. Sit in the chairs and watch the show (and maybe even enjoy it!), and you will help encourage audience members to stay and watch.
Audience members are scared to seat themselves during a show. People lingering in the back of the showroom cannot really enjoy the show and are more likely to leave. Just go up to them and say, "We have a table for you right here!" (be sure to have a table for them right there, too)
It should be easy to get other comedians to move off of a table during the show if you just explain to them that you want to give the seats to audience members.
This one is just a good look. You will be supporting the venue and building up the morale of the staff. The most important critic of the show, aside from the venue owner, are the staff. Nothing kills an open mic faster than complaints from the staff.
When you have your turn on stage, use it to tell your jokes. Do not use stage time as a soap box to insult or harass other comedians. It’s not funny and it will not only burn bridges but will also upset the audience, who wants to hear jokes, not shit talk.
If the audio levels are bad, or if the stage lighting is awful, notify the person running the show. If you notice there are not enough tables and chairs for the audience members coming in, see if you can help find some. Do not interrupt the show but do try to be helpful without being disruptive.