It depends on the size of the band concerned.
At the top end, you need advance acreditation from the record company or management, but a good way to start is to make friends with your local venue manager, and snap anything that comes through. This will get you some experience, and you can start offering to do shoots for bands. The law of averages says that sooner or later a new band will break, and if you have good shots, you can start syndicating them.
If you are easy to work with, professional, and reliable, work will come in, so be persistant, but don;t imagine you can turn up on the day and ask to go back stage.
Check out with the NME, Kerrrang! and similar, and offershots from the gigs you do - it's a good way in. Be persistant, but not obnoxious - if you are good, you will getr work eventually.
Good luck!