I like giving them "status" on the site or in the app. This is things like "Mayorships" in Foursquare or "Verified" on Twitter. The goal is to create a simple, exposed way that people can earn status. (This is similar to the "LinkedIn Connections game" that was played back in the day, you have to get to 500 connections but you don't want to be the person with their email address in their Name).
You want to be able to give them "superpower". See Kathy Sierra's Designing for a Badass User http://craftedresonance.quora.com/Kathy-Sierra-on-Designing-for-Badass
