IMO there's lots of good advice here. Another thought is to be an attentive sound-meister, especially for your less experienced performers. Make some adjustments for the different voices and instruments. Explain to folks, e.g., that it's a cardiode mic and they need to be right on top of it, etc. I've played at a number of open mics where the "host" sets the system so he sounds great, and then stands around drinking and bullshitting while others die a horrible death on stage. With a little experience and confidence, you can hammer your way thru a short set on a bad system and sound oke, but you can see this is killing the newer players.

- Merritt