Where did the prizes come from?
Are they ex review units your not allowed to ebay on?
I would expect the manuf to promote their own give aways otherwise.
If the motivation of the competition is to recruit new users, it certainly is a good incentive to get the lurkers to come forward and sign up.
If you want to reward active forum users, you could either pick thoughtful responses or interesting thread creators, and put their names into a hat, or possibly a forum poll for users to nominate.
Last idea, might be to select active users from past 30, 90, 360 days, and do a random draw.
I have no advice for me-too competition entries. I assume you log ip address, and could identify suspect names.
Alternatively an embargo on new user creation during competiton duration, however this would prevent any intended recruitment drive.
edit: this post probably belongs in the other suggestion thread. Sorry i mixed them up.