Helping People Lose Weight

I was thinking last night about doing some sort of health/weightloss gig. I could help people lose weight, but only if they actually listened to me. Most people appear to not listen unless they pay- of course, they may not listen even then, but at least I wouldn't be wasting my time. So, it seems to me it could be a fun thing if I could somehow charge less for the people who actually lose the weight and/or get healthier.
Tough to figure out. Charge a really high price and mark it down as they improve? Would I be able to get any clients at all with that model? If I could, they'd be highly motivated.
This is partly an economic insight, but economists are about the only people who'd actually make a bet in order to lose weight. Bet you'll lose the weight, and then you have a lot of incentive to lose it. Makes sense to an economist, but an average person is unlikely to make the bet because he didn't want that much pressure on him in the first place.

A person who contacts someone to help them lose weight, however, must be at least a little more motivated than the average person, but they need help getting used to and using non-mainstream ideas fast. A client who complies well and achieves his goals becomes the best sort of advertisement available, so one could regard the discount given as part of the advertising budget.

Perhaps it would function as pay X for month one, set goal. If you are 10% closer to goal by month's end, you get 10% off X. If you are 50% you get 50%. Now, excluding idiots who would starve themselves to avoid paying extra, it ought to work.