Win a new Blackberry, that's the draw used by a client to encourage visitors to complete a survey. As the end client is a maker of business management software, this is highly appropriate and if it weren't for the small advertising budget this would be an enormous success. As it targets the right prize for the audience, it will bring in sufficient data. So this win will work moderately well.
I have another client, Fandom Entertainment, who works with fan sites. This company's sites encourage new users to join and current users to continue to publish by offering signed scripts, DVDs and the like. Everything targeted directly to the fans of a given artist, movie, TV etc. The more closely targeted the better the result. Results diminish when just offering gift certificates. One would think that giving a user the choice of what to do with a prize would be a big draw, but it's the specific items that seem to matter more.
As I said, the BlackBerry survey is not highly publicized, so if you are 18 or older and already the owner of one and would like the chance to win one of the new models, have a look at Analystik's blog and pick the English or French option for the survey.
The closer the prize is to your product, the more promotion youd do for the chance to win, the greater the chance of success.