-be professional... start on time, don't go up on stage and stand around talking to each other trying to figure stuff out. Be all setup in advance, and try to keep the time from when you take the stage to when you start playing as short as possible.
-don't introduce "the band" before you play... instead, start with a solid, upbeat number, and if you can, transition from your first song right into your second. If you want, you can quickly throw out a "hey folks we're such and such, thanks for being here tonight..." but whatever you do, don't stop the music and start talking about each and every member and how it was hell to find a parking spot before the show. Keep your momentum going right to the end of your set.
-be yourselves... have fun, don't try to be "rockstars" but if your "frontman" (or front-woman) has an excess of personality, it won't hurt.
-never turn your back on your audience; don't turn to each other and start doing the Bon Jovi "inside joke" thing where you start talking in each other's ear and laughing... your audience won't like it.
-if its in your nature, be flamboyant. One story I really liked was about the lead singer of "Mother Love Bone"... he used to perform like it was a huge concert hall every time, even if it was just a small club with just a few people... "let me hear ya out there...! ok, now just all the guys in the back!!" stuff like that.
-uh oh, SOMETHING HAS GONE WRONG!!! Is it still going wrong and the song is a disaster? If not, just keep playing and let everyone get back on track, even if it means improvising. On the other hand, if something is really wacked (none of the mics are working), then just stop, smile to the audience and say "sorry guys, technical difficulties" and fix it calmly and quickly. Stay calm. Don't panic, don't get worked up. Just solve the problem and get going again.
-lastly, HAVE FUN! If you enjoy performing, your audience will likely enjoy the performance.
Good luck guys!
