One popular method has been the contest, which often ends up looking suspiciously like a lottery or sweepstakes. Sweepstakes are not legal in every state, and there are certain rules to be followed, certain phrases that must be included in every contest's rules in order to keep the promotion on the safe side of the law.
Such phrases should include "void where prohibited", "no purchase necessary", "full rules available at...", moreover, there should be alternate (write in) methods of entering, there should be a clearly stated start and end time and date for the promotion, the means of selecting the winner should be set forth. Ideally, there ought to be some skill involved to avoid the winner being chosen at random, but if the contest promoter satisfies two out of three criteria, he/she is probably fine.
Also, the contest promoter should be careful not to require "a consideration" (payment or a review or a "like" or some other valuable activity by the entrant.)
Here's a good guide: http://www.dca.ca.gov/publications/legal_guides/u-3.shtml
Here's another: http://contests.about.com/od/sweepstakes101/p/whatarecontests.htm
Here's a Thompson Coburn LLP law blog devoted to the topic:
The law may be changing, per the latter, for the FCC, but there is also the FTC. The following quote caught my attention.
the FTC brought an action against [a famous shoe company] for a sweepstakes promotion asking people to pin pictures of [the famous shoe company's] shoes, as well as destinations to which they would like to travel. People who pinned pictures received an entry into a sweepstakes. The FTC took the position that the mere act of pinning constituted an endorsement, and a sweepstakes entry was a "material connection that had to be disclosed." In other words, [the famous shoe company] needed to make sure that consumers disclosed that they were pinning pictures, because they were hoping to win a prize.Find the entire article on Lexology.com
All the best,