We did this at my school last Spring. It was great motivation (we had a large overall prize for the buy-in, but weekly prizes for the weekly biggest winner as well.) What was cool was that even though we were competing, we spent an awful lot of time encouraging and holding one another accountable as well as sharing tips and secrets. It was awesome. (And I won almost $500. We did it again this fall, and though I was inclined to not participate, I was encouraged to for motivation's sake. And I won again. This winter, I'm just managing the game.)
Good luck to you! The secret to my success was exercising during my lunch period. It sounds crazy, but instead of sitting in the faculty room complaining, I hit the fitness center the re-showered and got back to class. It was a great motivator, and by the end of the first month, there were lots of us down there sweating away. Sounds like a fast turnaround, and it was, but it was a great way to not let myself skip out at the end of the day because I was too tired or had something else to do.
"The grass ain't greener, the wine ain't sweeter, either side of the hill" The Grateful Dead