Everyone's asking for feedback these days. It's the in thing to do. It's good though, because systems with feedback built into them tend to improve by default. Which is why I keep drawing this picture a lot lately (B should be 'output' not 'outlook').
But not a look of people actually give users a reason to give feedback, or any kind of reward. Which is why I like this:
1: Create a simple, quick, attractive feedback form caturing just what you want to know (sex/age/media consumption/email address in this case).
2: Offer a nice reward to people who take the time to do step 1.
3: Honour that reward. Simple.
Feedback can also be rewarded by just listening to it in person with a note book and pen in hand, and responding to it in person. As Alan Rusbridger (Editor of the Guardian) did on multiple occasions on the weekend.