a) If you can, use a fast lens. Depending on the gig itself, you may find that the kit lens won't give you bright enough images unless you bump up the ISO right up, resulting in horrible grainy images. I use a Sigma 1.4 30mm prime which has served me well. The difference between footage filmed with this and other 550Ds with kit lenses at one multi camewra gig shoot I did was considerable.
b) +1 for the Magic Lantern Firmware, amongst other things it will allow you to film almost continously as it will automatically restart whenever you hit the 12min file size limitation (albeit with minor a gap of a second or two). But...
c) beware of overheating, especially if you are constantly gripping it (as opposed to on a tripod) in a warm gig environment.