The Room, a 2003 romantic drama from director Tommy Wiseau, is apparently so bad it's good. It's become a staple of midnight cult screenings, and not because of its high quality. As the film's Wikipedia entry so succinctly puts it:"The Room has been critically panned for its acting, screenplay, dialogue, production values, score, direction and cinematography."
How do you advertise the screenings, then? As honestly as you can.