Don't karaoke to the intermission music. Everyone's on stage setting up and fine tuning and the intermission/pre-show music is playing. Resist the urge to play or sing along with a song you might kind of know. Nothing good can come of this even if the drummer manages to stay locked in with the track. (Unlikely).
Don't start songs you can't finish. You're caught up in the wild abandon of a performance and somebody starts off that one song you almost had worked up at rehearsal. Bad move, because then it gets to that part that nobody quite nailed and a train wreck ensues.
Don't announce you're drunk on the mic. It's bad enough that you're drunk on stage to begin with. Don't confirm what everybody probably already suspects. And, in spite of what you've told yourself over the years, drugs and alcohol never, ever help your performance.
Don't talk over each other on the mic. One of my biggest pet peeves. Nothing says "amateur-hour" like three people interrupting each-other on a boomy PA. It's usually hard enough to make out what one person is saying on a PA, let alone 2 or 3 at the same time.
Never acknowledge another band member's fuck-up on stage. Be absolutely sure to acknowledge it afterwards, but on stage, never shoot a look, don't make a comment, do not roll your eyes, do not laugh. It's not funny, and you've fucked up plenty of times yourself. You know you have.
Don't turn the intermission music up louder than the band. Unless the band is not your band, and you happen to hate that band. If you're not confident that your band's music is entertaining the crowd, and you feel the need to crank up some boom-jiggy music in between sets to get the crowd fired up, you should quit live music and become a DJ. If you make the intermission music a "feature", you are greatly diluting the impact of your band. One of band-life's great humiliations is having a house DJ in charge of between-sets music, and you have to endure seeing the dance floor jam-packed with drunk chicks while the DJ cranks CeeLo Green or line-dance re-mixes. And then you get up for your set of originals...
Work out harmonies so you're not all singing the same note. A lot of bar-band musicians don't bother working out harmonies, and it shows. Improvised harmonies rarely work, and when two or more players decide to throw in on the big, sing-along chorus, they're often just slopping a unison on top of the lead. If you can't actually name the notes you're supposed to be singing, you're probably better off staying out.
Start a cappella songs on the right note. Very few people have perfect pitch. Get a reference note before starting an a cappella piece. Even the Eagles strummed a "D" chord before starting Seven Bridges Road.
Dress the part. If you show up to the gig looking like some shlub who just got done mowing the lawn, not only will you look like a fool, you'll have a tough time getting free drinks from the bartenders. Opinions vary wildly about what "dressing cool" looks like, but two things are certain, cargo shorts, sandals, or sweatshirts are never cool; black is always cool. Try to build a consensus on the band's "look". I've seen bands end up looking like a really mundane version of the Village People. "There's the auto-parts store guy. And he's the produce manager. That one's the substitute teacher. Unemployed stoner." etc. My best "dress-up" advise: you'll end up cooler having tried and perhaps failed than not trying at all.
Your soundcheck song/licks had better be fucking good. Most people (including me) will judge a band on their soundcheck licks and song. If they suck, I'm goin to another club. If there are patrons in the venue while you soundcheck, you must resist the temptation to "experiment". Don't play stuff you're "working on", songs OR licks. Only your most solid, well-rehearsed shit. If no patrons are present, get as Pink Floyd as you want.
Once soundcheck is over don't noodle on your instrument. Post soundcheck and pre performance is a time to build tension and anticipation for the performance. Don't ruin it by letting the pedal steel player putz around on his instrument for fun. This is "get-your-gameface-on" time.
Go by the set list. Few things are worse than hearing a band member say off-mic at the end of each song, "whaddya wanna do now?" And if too many people hear that too many times during the night, they'll begin to suspect that you don't have your shit together and they'd be right. Yes, there are times when calling off a song is appropriate, necessary even. But most times not, so when anyone says, "what do you guys want to play now?", say "The next song on the list".
Don't take longer than 6 seconds between songs unless it's a pre arranged "talk break". Many bands have one or a few spots between songs where they: thank the audience, encourage the tipping of waitstaff, introduce the band, talk about some social or political causes, or josh around while a guitar change is made. If it is not one of those occasions, STFU and tear through your set list. (see above)
Do not let drunk chicks operate/request intermission music. Again, treat intermission music as a breather from the main attraction, which is your band. If you're in charge of the intermission music, load up some "middle-of-the-road" classic rock or pop and put it on at a reasonable volume and tell people you have no control over the music. Hell, even DJ's quite often don't take requests. Make sure your tracks are "normalized" (i.e. have the same volume level.) (Apple products call it "sound check") Cell phones often make poor jukeboxes because their output jacks are not as robust as an iPod and the tracks probably aren't normalized. Make a few playlists and put 'er on auto pilot.
Don't insult the audience. Unless your shtick is being the "confrontational artistic aggressor" i.e. asshole. You might get away with it if you're Axl Rose or GG Allin, but you're not. They're washed up and dead, respectively. If the audience insults you first, this rule does not apply.
Wrap it right. For God's sake wrap your mic cables the right way. Loop and twist untill you have a nice round coil about a foot in diameter. Now secure it with a velcro cable tie. Repeat. And no, looping it around your elbow and tying it is not ok. Fucked up cables can definitely ruin a show. You must avoid general, all-around cable abuse at all costs. This includes knotting, dropping, twisting, kinking, yanking, boot-heel grindings, whip cracking the ends, and beer spillage.
Never pass out on the band RV while there are revelers about. Duh.
Learn to listen. Contrary to the laypersons belief, listening takes a good deal of concerted concentration. People who haven't learned to listen are the ones who keep turning their amps up after they're too loud already, or asking for "more me" in the monitors when your ears are already bleeding. Some people think that if they hear instruments other than their own, their own is not loud enough. You need to hear the other players. You're a band, for shit's sake. Learn to turn down. If you were on-stage with a national touring act, they are not very loud at all. Hell, lots of players these days are skipping an amp and going direct into the I.E.monitors/F.O.H..
The Drummer gets the last hit, man. For song endings, always look to the drummer for the last hit unless it's pre-arranged for another player to make the cue. In spite of all the idiot drummer jokes, they are an integral and foundational part of the performance and giving them the last cue will reduce "stray hits" from the rest of the band.
Play like it's your last show ever. And it's being video taped for worlwide broadcast. For your funeral. Rockin a crowd and being cool is why you got into this right? Try to play your heart out EVERY SINGLE gig. Remember you are trying to build a rock and roll escape hatch from this mundane world of dashed hopes and cruel boredom. Transport motherfuckers with your music.
Always have fun. No-one, and I mean no-one can sell a show if they're not having fun. If one guy in the band is having an off night, you can cover, but two or more is disasterous. If it becomes chronic, you all need to have a serious pow wow and figure out why this is not an enjoyable experience for you. I've said this to many, many musicians in my life, "honesty is the best policy." Advocate for your self and for your craft. You are the captain of your ship, man. Go forth and plunder!
There are many other online resources for the aspiring performer and recordist. DiscMakers is a cd duplication company that offers many services and help-guides regarding music. The Home Recording Guide, the Studio Guide and the Vocalsit Guide are particularly good. More stage performance tips here. and here and here and here and here and here. Here's an Open Letter from a Bar Owner to Musicians.