The thing with something like a free dinner is that most people's reaction is "I'm unlikely to win" (they're correct), so they're usually not that bothered.
Look for something that all users can get, such as thetrumanshow's suggestion of a "trendsetter badge", or something that a lot of people can get, such as "one out of every ten people to sign up will get a voucher to..."
Look for something that all users can get, such as thetrumanshow's suggestion of a "trendsetter badge", or something that a lot of people can get, such as "one out of every ten people to sign up will get a voucher to..."