I need some creative ideas... :)?

I am a moderator on a weight loss message board and I want to motivate everyone to do a 2007 Weight Loss challlenge. Do you have any ideas on how to do a challenge like this online? Should I get people to post their weights online or just the amount lost each week? I know some people may be shy at posting their weight where everyone can see. Should I have people check in daily or weekly or report to me? Or should I form teams? Any suggestions would be great! Oh and I can't afford any prizes for the winners. Any suggestions on this would be appreciated. Thanks so much! :)