It’s harder than you think! I’m considering starting one for my upcoming book The Secrets of Top Students: Tips, Tools, and Techniques for Acing High School and College and I highly recommend this checklist provided by Santella and Associates. It’s chock-full of information about what’s legal and what’s not. (Hint: most things are not technically legal.)
Here are some other good links:
Legal Issues Affecting Promotions and Sweepstakes
Social Media Promotions and the Law
How to run a website contest without going to jail. This one’s particularly useful for writers.