The laws governing contests are different in each state, and there are federal FTC and consumer protection and fraud laws that apply also, so you can't assume "the 50 states" for your queries without a great deal of legal work. You have to comply with the laws in each state where you plan to offer your online sweepstakes, so this isn't something you can do yourself. For example, Rhode Island actually requires the filing of a legal statement before you can run a contest in that state, and noncompliance is a criminal misdemeanor.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
As noted by Attorney Koslyn, the federal government and all fifty states have established their own rules that apply to lotteries, contests, and sweepstakes. I know of no template set of contest rules and sponsor conduct that satisfies all of these various laws. Two decent articles on the subject are here: http://j.mp/dw2x0k and http://www.dmaresponsibility.org/Sweepstakes/ . You should also query Google for "sweepstakes attorney" and hire one licensed to practice in your state. Good luck.
Your question requires more detail than expected by avvo attorney responses. Each State has their own rules regarding sweepstakes, lotteries, etc. The responses given are great place to start.