Have you heard about gamification. Adding badges and such to your site to get people using more parts of your site in order to earn rewards and work towards goals like badges for good attendance, most comments, etc.
It’s likely you have already been on a site which uses this technique. What do you think about it? Does it make you feel belittled, is it addictive or is it just simple fun?
Website Magazine: How to Gamify your Website
Mashable: How to Gamify your Marketing