Don't play and leave
I must say I see a lot less of this these days and that's no bad thing. Now, I don't want to be all fascist about this, I understand people are busy, people have lives and people cannot always stay until the end of the night, but generally I think it's pretty cool to hang around after you've played for at least a few acts. So many times I've seen people turn up, talk until they're on, play, and then bugger off - as far as I'm concerned, that's just plain B.S!
Again, don't want to be a nob about it - sometimes your mates come to Open Mic, sometimes you do a lot of talking, we all do, but I think everyone should make the effort to at least listen to some other people playing. This reciprocal arrangement works for everyone and makes Open Mics cool - don't expect people to listen to you if you're going to sit there nattering away all night. That too is B.S!
An Open Mic is an Open Mic
That means that anyone can get up and do whatever the hell they want. I've seen people (not much thankfully) 'clapped off' just for doing something a bit abstract and I've seen one too many snidey glances at some performances.... and don't get me wrong, I've probably been guilty of this as much as the next person, but it's something I think we should all guard against. We should all embrace the spirit of Open Mics and all the diversity that comes with them. Okay, soap box moment over.
Buy a drink
Yes, times are hard and drinks don't come cheap, I appreciate that. The thing is, I used to know this one guy who went to the same Open Mic every week (on their own) and all he ever got was a glass of free tap water from the bar, that is just B.S. methinks. Open Mics exist so pubs can make a bit of cash out of a quiet night - I mean whatever, if you don't want to buy anything then that's your choice, just don't go moaning about it when your favorite Open Mic comes to an end. Like it or not, Open Mics are as profit-sensitive as everything else in this world.
Anyway, like I say, ignore all this if you want, it's just my thoughts.