Make extra posters. You will often find a few extra places to hang them, and sometimes they will get pulled down or defaced, and you will have to replace them. Radio advertising is effective and not too expensive. Ask about special package prices. TV advertising is usually too expensive for small shows, and not noticeably more effective than radio.
Some cable systems have inexpensive advertising space on a bulletin board channel.
If the sponsor is a non-profit organization, local public radio stations may give you some spots. Even commercial radio stations sometimes will let non-profits use their community announcements program. Also, try to arrange a live radio interview with the main performer on the day before or the day of the event. These can be done in the studio, if the performer is already in town, or can be done by phone from wherever the performer happens to be. They are most often possible on public radio stations.