I shot a battle of the bands in a similar location, but I didn't do it that well...
things that I was glad I did/would do differently:
Make sure you get a recording of the audio off the sound-desk, although the levels will be all out you can combine it with the audio you record live.
Set up a decent microphone or two to record sound.
get a wide-angle lense for the close-up shots.
get plenty of cut-away shots of the crowd and things, from diferent angles,
bring as many spare batteries and tapes as possible, cordon off your tripod area and keep everything in arms length.
get there as early as possible to set-up, talk to the band/sound guy/management etc. they will probably be able to help you out, if anyones recorded a gig before there they will have talked to the people there so you can ask how they did it.
get some fans/crowd thoughts afterwards. always good to go over the credits or as a bonus.
take a stills camera (even just a small digital one) to get some pictures for the DVD cover/disc artwork.
But I've only filmed a live gig a few times, there's people on here who do it professionally, I'm sure they'll help out.
Panasonic SD9, Panasonic NVGS-75, Canon MV600i