There is an interesting article over MaintainIT called A commitment to patron training. Sarah Washburn discusses how they have got people to come to computer training by having a small charge of $5, which is then refunded to them once they start the course.
A very neat idea, as most people don't mind wasting time but hate wasting money. The person who started this Claire Stafford says of this scheme:-
“If they come to the classes they get their money back. We did start it as a free--non-gratis--service, but we were finding that if people didn’t make a commitment, that they were not coming in. And we had waiting lists that were just tremendous! And we couldn’t serve these folks that were waiting, and the folks that did sign up forgot, or had something come up, and failed to contact us and let us know. “