Challenges are a great way to get people going.
What I always suggest people do is to do a diet specific challenge matched with a picture transformation challenge.
For example, I ran the One Big Meal Challenge. Very specific eating but also picture specific. The winner is the person who has the best transformation, not most loss, but transformation. Who complied to their diet best, etc.
Make it involved, get the family talking about it, make it something that brings praise and poking if you mess up. Pride trumps money everytime if triggered right. Here is the details of that challenge to give you an idea.
http://avidityfitness.net/2008/10/17...-one-big-meal/