I have hosted this same kind of event 3 times now. We weighed in EVERY Friday (as to keep people accountable) We also have people pay if they skip a weigh-in ($5), and also pay $1 for every pound they gain (rounded to the nearest pound). The first challenge the prize pool exceeded $450.
I am starting a new one in a week. This is going to have a points system and participants will earn point for daily activities. Once they get to a certain amount of points, they will receive a small prize.
Just keeps things interesting.
Good luck! let me know if you need anything else!