Ask HN: How to create a successful Hackathon?

First time organizing a hackathon for the upcoming SocialDevCamp Chicago (http://socialdevcampchicago.com). Lots of interest expressed in the event but struggling to get pre-registration for the Hackathon itself (to prepare network and get server space if necessary). Any HN suggestions for encouraging attendees to bring ideas out of the woodshed for the weekend? We have prizes, experienced coders available for guidance, and flexible guidelines already...Thanks.