You can use this strategy not only for immediate results, but it can also be very effective in link building to your site and developing a mailing list for potential leads.
Here's something you may find very interesting:
http://searchengineland.com/a-case-study-using-contests-to-build-links-23776
Here's something you may find very interesting:
http://searchengineland.com/a-case-study-using-contests-to-build-links-23776