5. A trumpet player is not a good side man at an open mic.
6. The only true wisdom is humility. (T. S. Elliot)
7. Don’t start over–they aren’t listening and didn’t hear your mistake.
8. Don’t get angry when you screw up. It’s the audience who deserves to be angry.
9. Don’t bring written lyrics to hold while you perform. If you don’t know your song, why would you make people listen to you practice?
10. If you do a duet, make sure you understand the nature of that relationship. Trios are safer.

And I would also add:

11. Don’t tell the audience ahead of time that you are going to suck. Disclaimers are for suckers only.