What makes a good Hack-a-thon?

Hi HN Community, I'm trying to organize a hack-a-thon for chi-town (that's Chicago :))and wanted to know what makes a good hack-a-thon? And what format would be the best to start with? I am the community leader of the Chicago Java Users Group, but this could be across any language/platform. Also, if you are in the area and know of someone who can sponsor (space/time/or volunteer) it would be great!

Thanks!