Folk clubs are generally very poor at reaching outside the established folk scene. It takes a lot of leg work as Ian found and then you need to sit down, look at the results and decide where to advertise next time. When people come in off the street you need to talk to them, find out how they heard about the club. If a newcomer turns up on a singers night then take the trouble to explain how that differs from a guest night otherwise when you are trying to sell a gig with a major name they are telling their mates that you just run the musical equivalent of am-dram.