Duff Robison's guide to open stage etiquette:

1. Be gracious.  You will meet people from all talent levels.. Some are just starting out, and some have been doing this for decades.  Be kind and encouraging to everyone. You will be amazed how even the greenest musicians flower and improve given the proper advice and encouragement. 

2. Check your ego at the door. There will always be people with less skill and talent, or more skill and talent than you have. No one wants to listen to you complain about the lack of monitors, or the sound, or sit there and slag another artist, or go on about how great you are. Be a good human being and work at encouraging everyone who wants to play live music. We need more of it. Don't be a douchebag. We need less of those. 

3. Don't show up drunk or stoned out of your mind, expecting to play.  This one's pretty simple. A good host won't let you up to play if you're visibly inebriated. Period.

4. Be fair. Don't text the host and ask to be "put on the list" and expect to walk on when you get there. This is bullshit.  I've been to lots of open stages where it was the host and their buddies who got to play, and everyone who came down to put their name on the list, spend an evening patronizing the bar, never got to go up, watching the host's buddies walk in and go on stage. This is crap, and in my experience, those hosts never last. The only only fair format for an open stage is everyone goes on the list in the order in which they arrive. Everything else invites nepotism and hard feelings. Also, if there is a song or time limit because of a full roster, don't ask for more time or special consideration. Everyone is waiting to play, and everyone deserves equal consideration. If you think you're more important than anyone on that list, maybe you don't belong at open stage. Maybe it's time to go on tour. 

5. Never go on stage to join another player without being invited or asking permission. This is common courtesy. A player in the middle of a song may not want the help, no matter how much you feel you may be able to contribute. I've seen many performances ruined by a well-intentioned cajon "player" or misplaced back up vocalist. Obviously, if you know the artist, and you have a history of playing together, that's different. If in doubt, always ask. If you sense hesitation, back off. Open stages are about meeting other musicians, learning from each other, and jamming! Most players love to play with other people, but not every act needs an unrehearsed "drop in". Many new players are understandably nervous, and an uninvited guest on stage will only throw them off.

6. Don't show up at 11:00 or 11:30 pm expecting to play. Or even 10:30.  At most established stages, it just ain't gonna happen. Sorry... But please come back and come earlier... we'd love to have you!!

7. Please tune your instrument. If you don't have a tuner, most competent hosts have one you can borrow. Seriously. Tune.

8. Don't show up to sing and ask if someone can play for you. Sorry, this is not karaoke night. Not that there's anything wrong with that. But it's not.

9. Invest in an instrument with a pickup, or a pickup for your instrument. If you are are frequenting open stages, don't expect a microphone for your instrument. That rarely works very well, let's face it. Don't get me wrong, in a studio environment, it's great. But at an open stage, a pickup is best. If you're not using a pickup because you're being a purist, get over it. It's open stage, not Carnegie Hall. If you're not using one because you can't afford it, maybe ask to use the host's instrument, which should not be a problem, if they are a proper host.

Conversely, here's what you should expect from a good open stage host:

A proper greeting.
A clear indication of where you are on the list.
A clear indication of how much time or how many songs you are allotted.
A proper introduction.
Attention to what you need for equipment, like a stool or music stand, a pick, or capo. Seriously, this is what they are paid for. 
Attention to your sound. This would involve actually running your sound, not abandoning you and going for a smoke.
A proper thank you when you're done. 

Most importantly, have fun and make connections. So many groups have had their start at open stages. Enjoy!!