"Biggest loser" type challenge question...

I do one of these at work twice a year (both administering and participating). It's a 12-week challenge. Everyone weighs in at the beginning and the end on the same scale. Participants get prizes based on the % of their initial weigh-in that they lost. The brackets are 4% ($5), 7% ($75) and 10% ($100).

Then to prevent yo-yoing, if you join the next challenge, the % goes down and the prizes go up, BUT your initial weigh-in is automatically your final weight from the last challenge. So you have to maintain your loss in between or else have a lot of damage to undo in order to win another prize. Subsequent prizes are 3% ($100) 5% ($200) and 7% ($300).

It's company sponsored and we don't pay into it, so obviously you'd have to figure out your own prize structure.