Legal Question About Website

Hey guys, I am thinking of starting a new website and need some assistance and suggestions on whether it is legal in the United States or not. Here is how it will work:

The website is called Win50.com and it will be a message board (the actual content of the message board is undecided). However the "Win50" part comes in because if a user posts at least 50 messages a week consistently throughout a whole month (or something along these lines), then he has the chance to win $50. Now I need some assistance in deciding how I can make it legal and not violate lottery/gambling laws, in terms of how the user has a "chance to win $50" and is not guaranteed to win that money but only does so if he/she is selected or something. I need suggestions on how to reward someone (with money or a gift certificate or something related to "50")LEGALLY if they post more than 50 posts a month.

Thanks and all feedback is appreciated,
Matt