For habit forming, look to beeminder - its interface is clunky and its termonolgy too geeky, but it is also the best way to make "some day" to day (some day I want to lose weight) and keep it up that I have found.
Thanks. That's one avenue we've looked at already. I have also looked at traditional gamification techniques and found them initially motivating for users, but then engagement falls off a cliff. This has less to do with the techniques I suspect and more to do with the fact that people eventually hit a goal weight and think they are suddenly "fixed" and can go back to what they were doing before. We actively work to encourage the better mindset of making a permanent change in lifestyle where our system then moves from educational and informative to simply a tool to continue to plan and keep on track.