Research the band. So that you know how many there are going to be and what they play (important if there is a 'leader') maybe look at YouTube videos
Have a look at the stage set up before the gig starts, so you aren't blocked by pianos
See if you can get a shot of the setlist, so at the least you know what the first three songs are
Be where you want to be for the first song so you don't lose time getting there
Manual exposure, unless you are very sure of spot metering
Chimp or use mirrorless to check exposure as you go
Focussing - I tend to use single point. Rarely use manual unless the performers are very static
Watch the performers for interesting things - try and capture signature actions. Often they will repeat actions - but don't waste a song waiting for one
Smile at them if they look at you

Try to have performer's eyes open. Some performers don't smile, have eyes closed - that makes it hard!
Take two bodies if possible - three songs gives you no time to change lenses. Wide and narrower.
Get each of the performers, and groups of them and the group as a whole
Don't forget the drummer
Try to keep your other eye open watching what is happening outside the field of view of the camera lens. Realising the guitarist is now on the floor standing next to you too late is mortifying

If there are other 'togs there, treat them with respect. Don't hog the best position, and make sure you aren't in their way/standing on their toes etc.
If there are security at the front, engage with them.
If there is a 'tog pit, when the three numbers are up, go.
If there is manic lighting (I'm thinking Gong) burst shots are about the only way of getting something usable.
Ear plugs. Really important... plus some venues insist you have them.