I have taken photos at gigs and concerts a few times, but only from the front of the standing section. What's the best way to get a photographer pass?
Best place to start is by contacting local venues and promoters - it is unusual to be allowed to use flash, and also unusual to be allowed to stay in the press pit for the duration of the gig. It's definitely worth putting together a portfolio, which could also include doing promo shots for local unsigned acts - they may also be able to get you the press passes you want as their guest in return for rights to use the shots.
Don't forget to take advantage of any free arts festivals for building your portfolio -- there was one near me the other weekend where almost 40,000 people came along to enjoy the music spread over 7 stages, all absolutely free of charge -- amateur photographers were allowed side by side with the professionals, and were happy to swap tips as well.
For a given concert, I usually look up the band web page and find the promoters or agents. I send them a quick email, and attach a few of my concert photos I consider "the best". I assure them I will get in touch shortly after the concert with the photos (say, within 4 days), and that I won't use the shots for commercial purposes unless I have the appropriate permissions.
If we agree on something I close the loop a few days before the show to check that I am on the list; trust me, you don't want to be stuck at the door. I wrote some more at the end of that post. I advise you not to delete too many of your photos during post-processing, just in case the promoter gets back to you looking for a specific angle. Building a good relationship here is key, since that band may come back in your area in the future. Better yet, that band might later on open for a much larger, difficult to access band and provide you with a precious press pass...
Ditto about the free/public concerts, it's a great way to build your portfolio.
Go to smaller gigs within the scene that interest you. Get to know the promoters. Offer to take photos for the venue. Expect to get in for free, but don't expect much payment - smaller venues (at least here in the UK) are mostly just getting by as it is. Pass the photos to the bands for their MySpace / Facebook / whatever.
From there, network! If you're good, people will see your work and you'll build a portfolio. Once you're confident you can get the shots, approach the bigger venues with the bigger names. Try a local newspaper - if they run live music reviews, they'll have photo-passes pre-arranged. Show them your portfolio and hope they put your name on the list. Be available and reliable. Don't be put off if the first one doesn't want you - they only keep so many on stand-by at a time. Try again in a few months.
From there, the photos get easier because the bigger venues have better lighting and acts with money to put on a show, but there's more pressure. If you come home without the shot don't expect another call any time soon!
That's pretty much where I am at the moment, and it's fun. Good luck!