Jonothan is right. I used to do this on a "professional" basis and whilst I sold pics to the press, I still found myself needing to write, DJ and live in a squat in order to make ends meet. I got out of it eventually!
Regarding pictures, you have chosen one of the hardest subjects in the world. You need everything - including luck, on your side.
Lose the flash - they are usually banned in bigger venues anyhow and are f**king distracting when you are performing - not a good way to indear yourself to the band. With the luxury of a usable 3200 ISO in digital, you don't need it. I used to push the hell out of neg film. Aside from that, unless used very sparingly (as fill in) it sucks all atmosphere from the shot. (Do experiment with fill in flash once you know the band and have explicit permission)
Learn to use flash! www.strobist.com
Bands always need promo pictures and these tend to be shot away from the stage. They are also where the ££ is. If you can get a gig assisting a pro either in a studio or on location, you'll learn a whole world. On the same note, make friends with people doing makeup courses. They are dead handy. They also tend to be attractive - which helps if you are doing this for the same reason I did!
I used the spot meter function on an EOS 1 (film) to judge exposure. You will find that just as you got your reading and dialed it in, the state changes. You do it all again, the same thing happens. Plenty of practice will teach you how to make a very educated guess. I suspect that matrix metering modes have come on leaps and bounds since I was doing this stuff. If that is the case, I'd work from -1.5EV and use shutter priority mode.
Shoot RAW. Colour balance is a nightmare - this will give you a fighting chance. Theory has it that with RAW, you can ALWAYS get a perfect WB after the fact in post. I don't think this is 100% true when you are shooting in extreme colour casts. i think it's probably worth learning to custom WB was well as shooting RAW. A cunning photographer would ask the lighting tech what colour temps are used and store them on custom functions before the gig.
Get to know bands real well. Go on tour with them if you get the opportunity. If they will allow you to shoot on stage, you are away. Use a 28mm lens, get in close and low and stay out of their way and you will be rewarded with some great shots.
Learn to spot talent. If a band's music is crap and they have no stage presence, they are a dead duck. No one wants to buy picture's of your best-friend's boyfriend's navel gazing, going no where band. Get intimate pictures of "the next big thing's" first tour and you might just find you don't need to eat lentils every night. Don't let your friendships override your judgement on a band.
Think of a memorable photo of Ringo Starr or Krist Novaselic. Can't? Don't spend too long shooting pictures of drummers and bassists - it's a fickle world and frankly - the press don't care! John Lennon, Paul McCartney and Kurt Cobain were were the money was. No one cares who provides the rhythm - sad but true. NOTE - if they are doing something that could be considered "rock n roll" or, alternatively, "obscene" - scratch this rule. Even if they are never going to make it, you have good material for bribing them.
If you start shooting larger venues/festivals, things get way harder. Usually, you get 3 songs "in the pit." If you are first in, it's even harder. Organise your gear and get the lighting figured out before you are in. Play conservative rather than risk everything. Be polite with those you are sharing it with but don't let them intimidate you. You have the same pass as them.
Endear yourself to gig promoters and venue managers by whatever means necessary.
Edit really, really hard. You need to some up a gig in ONE photo. One picture to cram all of that passion, emotion, fury and elation. If you do that, you did well. If that's the one your picture ed chooses to run with, it's time to celebrate!
Apologies for the dust on these!