OK. You might think these rules only apply to a certain member of the band, but they apply to everybody.
Smile! Unless your band isn't supposed to, which is IMO really lame. People want to have a good time, and they want to see you having a good time too.
Acknowledge the audience: You need to let them know that you know they exist. Audience participation is a great way to do this. Play short games and contests with them.
Have a lot of fun on stage: Even if you aren't having fun, try to make it look like you are. The audience is watching you. If you aren't having a great time, neither will they. Run around, dance, jump, sing to audience members, point at them, get off the stage and run around, whatever! Don't make it look like you feel obligated to be there.
Keep the list a mystery: Don't let the audience know what's on your set list! Keep them in suspense, waiting for that special song. They might otherwise wander off and do something else, or stand in the back talking. You want their full attention.
Following the set: If you have instructions written on the set list, pay attention to them so you'll know exactly where you are in the show.
Instructions on the set list: If you have instructions written specifically to you, follow them so the rest of the band isn't embarrassed or confused.
Deviating from the set list: Make sure the rest of the band knows about this, without making it immediately obvious that you have to make a change.
Inside jokes: This is a big no-no on stage. Refrain from them at all costs! If you tell some joke that would embarrass someone in the band, and they don't enjoy the joke, you just wrecked their stage presence. If you're the only one that thinks something is funny, keep it to yourself! This includes alternate names for songs. A horrible example is one of my band's songs, which we codenamed "The Poopy Pants Song" between the band members. Don't reveal that sort of thing to the general public, it's embarrassing to the whole band, the staff at the show, the other band(s), and especially your management.
New songs: Expect to screw up a lot. Say that it's a new song, and that you haven't played it a lot. Unless you want it to be a surprise, of course!
Screwing up: We all screw up at some point. Even the best of us. When you make a mistake, get back on track as smoothly as possible, even if it means waiting for another part of the song. Remember, it needs to be funny to you. Smile! And if someone else screws up, don't shoot shocked or confused glances over at him. He is already embarrassed, and if the audience hasn't noticed yet, your sudden reaction will bring it to their attention. If it's a new song, keep on playing like that's how it was supposed to be.
Different audience, same show: If you play in front of a totally different crowd than usual, don't apologize for the way you sound in any part of the set! Don't be ashamed to show the world who you are. Play the same as you would for any other crowd.
Different mood, different show: If your audience is not in the right mood, don't apologize or stop the song. Try to formulate some type of alternative set list if the mood dictates.
Train wreck: This is the worst nightmare of every band. If you train-wreck a song, it's funny to you. Even if it really isn't. Make a joke about it, and say something like "OK, we practice a lot..." Then continue the set. In some cases pick up the song again if it's possible to do so without making the song feel like it's dragging on.
Repeating a song: Don't do it right away. Play something else first, or it will grow tiresome.