Picture
You need to have a good video quality, it doesn't have to be HD but nowadays HD Cameras are getting cheaper and cheaper. make sure the cam has either the possibility to shoot in good quality with a longer exposure or has a good nightshot (i suggest SONY, cause they have 0-Lux nightshot, which means you can shoot in total darkness and still see some people) also if you are not able to hold the cam steady while filming, buy a monopod. thats like a broomstick that you can attach to your cam, it will improve the stability of the picture a lot.

Sound
Because its a LIVE video, a decent sound is absolutely mandatory. theres no use of a good picture if the sound is just a mash of bass and the occasional cracks. If your cam has a port for an external microphone: GET ONE. they arent very expensive (i paid 150€ when i got mine years ago). it will improve the quality of your video massively.

Zoom
Don't overuse your zoom. mostly a total shot is the best choice, capture as much action as possible.

Placement
check the surrounding before you start shooting and think about the best location. if you have access to the stage (i.e. the people in the band and the securitys, if there are any are ok with you being on the side of the stage): get there and shoot from there! you'll have a perfect view of the band and the pit, also you might even profit from the monitor sound. in front of the stage is NEVER a good place to be, mostly because thats where the action is and if you film the band you'll have a face and the ceiling and thats shit. if you have a monopod or trypod even, you also may choose the soundboard as your location but make sure that the soundguy is ok with you being there. if none of both is accessable to you, you will have to find the next best spot. sometimes you wont be able to do anything about and shoot from the crowd.

"the pit"
If you are filming the pit, dont zoom onto people, no one on the internet cares about your stupid friends' faggot moves. get a total of the whole pit or a part of the pit if that is where the action is best. don't zoom on their faces etc... if theres a fight, ZOOM! don't stop filming! there will be enough people trying to break it up so you might as well try and catch as much of the fight as possible.

"the band"
use the zoom to get details of the band (drummer, singer close up etc) but dont do it too often and not if the band is a very energetic one. rather get the action.

"shut the fuck up"
theres enough people in the room making noise already, no one wants to hear you sing the wrong way, talk shit or anything else.

be thankful!
always at least try and give the band a copy of the show in the best possible quality! if necessary burn it onto a disc and send it to their home.

what when the show is lame?
well either stop filming and delete it again or if you chose to continue, try to get good footage of the band, maybe your stuff ends up in a live-footage-clip or on a dvd of the band.

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if you're asking me know who the fuck i am to give you tips on bootleg and/or tell you that your videos suck: i've taped HUNDREDS of shows all over europe, from small unknown bands to world famous hardcore bands. i've had clips from my videos published on band-dvds and live-compliations.