[kuu]

I participate in a poll/survey portal as a reviewer. Just to give you an idea of how it works: we the participants are free to register and get some points after every completed poll/survey (the amount of points depend on how long it took). With these points we can access to a "shop" of small prizes, like headphones, small cameras, books, etc. I assume that the companies making the polls are the ones paying and probably they select the demography of the people to survey (we're asked about this info on the polls). Maybe a model like that can work for you too.