I was a member of a message board that was more or less a random board. We ended up adding a thank you tool to the board so if you found something useful you could thank the post. It didn't take long for the more helpful members to rise to the top of the most thanked list. We also had board stats at the bottom of the main page that showed top thanked, most viewed, etc. as the board slowly died down the thank you add on started to become a popularity contest, but on a site of this size I think it would work well.