Coming to the technical part now. It's almost impossible to shoot these gigs without a fast prime and high ISO. Do not be afraid to use the highest ISO setting for 90% of the show because you'd rather have a noisy shot, than one which has a lot of motion blur. Also cheap primes like the 50 f/1.8 will not focus in such low light and APS-C cameras do not have a great viewfinder to nail the focus manually at f/1.8 each time you try. I prefer Live View shooting combined with manual focus. The drawback to this method is that you end up sacrificing a few stops on the shutter speed as holding the camera in the Live View mode as opposed to the traditional method induces a lot of camera shake. Also, do not be disheartened if you've shot 400 images and got only 30 usable ones. You've got to shoot at least 2 images of the same scene to make sure that you've nailed everything correctly. With practice, comes perfection.
Most of these gigs are not well-lit and the ones which are, have extreme colours on stage. These could end up spiking your RGB histograms making few areas of the photograph unrecoverable. So, you've got to keep on eye on both histograms after most shots. Oh and it is pretty obvious that you HAVE TO shoot in RAW if you want to have any sort of headroom whilst processing the pictures. Shoot in Manual mode at all times as the Av/A mode will give unusable shutter speeds for most of the time and every other mode is completely useless for indoor concert photography.
I feel that one must also know the music to shoot the music. A person who has no idea what the heavy metal subculture is all about, will have very little success in shooting a black metal gig and the same can be said about a person who's not fond of Indian Classical music and is shooting at Sawai Gandharva. One has to anticipate the emotions behind each riff, the mannerisms behind each beat and the showmanship of every individual to capture it at the right moment. Here are a few photographs from the various gigs that I've shot so far: