Record a few rehearsals
Even if you record on a boom box, we recommend that you tape at least few rehearsals before your gig. (Many successful bands record every rehearsal.)
Listen back with the whole band, and talk about things that you can improve on: Is the band tight? Are the parts played by various instruments working together? Can the arrangement be made better? Work on song endings -- make sure they're solid.
In general, less is more: simplify parts to make them easier to play. Simplify arrangements to make the song more catchy. Create more space in the music so that the instruments can be heard more clearly.
Strings and drum heads
Use new strings and drum heads. Your instruments will sound brighter and punchier. Most album recordings are done with brand new strings and heads.
Watch out though -- don't change strings and heads on the day of the show, since they'll likely go out of tune. Instead, change them two or three days before, and break them in.
Cables and equipment
Make sure you have working cables. Check amplifiers for hum and crackling noise. The less noise in your instruments, the cleaner your recording will sound.
Tuning is very important. Your guitar, bass or saxophone may be only a little out of tune, but this can make a big difference in the perceived quality of a recording. Tune, tune, tune and tune again. The same goes for guitar and bass intonation -- make sure that your intonation is adjusted properly. If you don't know how to do it yourself, have an expert set your intonation for you.
Keep your volume under control
Recording live presents its own specific set of problems and variables. Often getting proper isolation between instruments can be difficult, and amp settings can make a big difference.
Don't play too loud
The louder you play, the more sound from your guitar or bass goes into the drum microphones, and the tougher it will be to get a clean sound at mix time.
Generally, you shouldn't play with too much distortion, overdrive or compression. This goes for even metal and hardcore music. The dirtier your guitar or bass sounds in your amp, the harder it will be to make a mix sound crisp. The more compression you use, the harder it will be to make your mix punchy. Only use as much distortion and/or compression as you need to get your sound.
Don't hit the drums too hard. Hit them only as hard as you need to, in order to get your tone, and to be heard by the rest of the band. The louder your drums are on stage, the more difficult it is to get decent isolation in vocal and/or horn microphones, and the muddier your final mix will sound.
Don't noodle between songs
Noodling makes it tough for us to get beginnings and endings to sound tight, and it distracts your audience from your performance and your music.
Noodling between songs also makes you look less polished -- even unprofessonal. After all, when was the last time you went to a stadium concert and heard the bass player doing the theme to the Get Smart while the singer was talking to the audience?
Get the word out
Remember: You're recording live. Live recordings will sound more exciting with more audience sound, so get as many people to the show as you can. Tell them ahead of time that you're recording, and to make noise.