My other job is location recording and one of my favorite ways to add room/crowd ambience is to put a figure 8 mic at the edge of each side of the stage. You get a nice stereo fill from the stage, plus sound from the audience. If placed right, the PA and the monitors end up in the null spot.
Another favorite is an m/s pair at the mix position. I recorded Zilla (
www.zillamusic.com) at the Fox Theater a few weeks ago and the m/s consisted of a Beyer M260 in the mid position and a new CAD ribbon in the side position. It sounded great! I use 414s for this a lot and the ribbons gave a very similar sense of space but much much smoother. These were borrowed mics, but I liked it to the point that I had to I go out and get one of the CADs for myself. I did a rough mix during the multitracking and they liked it so much that it's available for download already!
If you are unfamiliar with m/s stereo mic'ing, check out
http://members.tripod.com/~offbeat2/ms/sndms.html
It's a great technique and allows you to make stereo decisions during the mix.
Edwin