Give away something to the winner of the contest as a motivation to join. This has been a proven method to get new users to try your website.
To get the world out, find a popular blog related to the topic and offer them exclusivity on the contest information.
Case Study:
Speek, one of the best conference service out there, outdid everyone when they hosted an event where you could enter into a contest to speak to Edward Norton, the famous hunk who starred in Fight Club and American History X. Here's the event page: http://blog.speek.com/site/win-a-speek-conference-call-with-edward-norton/
