Two different folk clubs in my area do basically the same thing: they ask the performers to e-mail or snail mail, in writing, exactly what they want said about them (verbatim!). One club uses the performer's blurb in a newsletter announcing the upcoming showcase; the other club uses the blurb in the verbal introduction by the emcee.

One "don't" that I saw the other day: Don't stand in the middle of the stage to introduce the act if there are stage hands attempting to set up mikes around you, and if you're hindering the performer in any way. If the stage is large enough, do stand to one side of the stage. If the stage is small, do stand at the front of the stage so that the stage hands and the musician are setting up behind you. In either case, a separate mike dedicated to the emcee is best, so that the sound folks don't have to readjust the levels on one mike every time the emcee uses it and then readjust again for each performer.