I've been of doing another open mic recently and found my notes from last time. I thought I'd share some tips from what I learned.
What I learned at the open mic:
1. Any music sheets you bring, make sure you can read them in any light and have large enough print. (I had no black ink at the time and couldn't see the blue text in the dim light)
2. Memorize as much of the songs as possible. (I forgot many parts and would have benefited from remembering how they went better)
3. Prepare and practice the songs way long before hand, and choose which ones you feel most comfortable with - if covers make you feel uncomfortable, don't do them. (I did a mash up if two songs which I more or less butchered because I didn't practice enough)
4. Sit cross legged, your nervous jitters will be less noticeable. (My legs were shaking like I was trying to beat eggs with my feet. Sitting cross legged made it seem I knew what I was doing and that I wasn't scared shitless)
5. Have the mic at a comfortable height. (I had it too high at first, and was too nervous to readjust, but it makes a ton of difference for performance and confidence if it's the right height)
6. Bring water up with you. (My nervousness made my mouth dry. Water became my only thought after 'gah! I can't believe I'm doing this!!')
Anyway, hope those tips help. :)