Some points from experience:

Advance sales means cash in your hand not promises. We always had problems with club organisers who would say they were bringing 20 people from their club and turn up on the night with cash for 5 sold tickets and 15 returns. (That sort of mismanagement caused me to resign from that committee in the end)

As Craig says get ads out in good time. If you have a regional folk magazine make sure that you time things to hit their press date.

Don't under sell the gig. If you charge significantly under the going rate the punters will assume that the quality will match.