I hope this can help. Btw, here in Brazil this is illegal. A good number of fashion blogs already payed money because of this here.
FTC Publishes Final Guides Governing Endorsements, Testimonials
http://www.ftc.gov/news-events/press-releases/2009/10/ftc-publishes-final-guides-governing-endorsements-testimonials
"These examples address what constitutes an endorsement when the message is conveyed by bloggers or other “word-of-mouth” marketers. The revised Guides specify that while decisions will be reached on a case-by-case basis, the post of a blogger who receives cash or in-kind payment to review a product is considered an endorsement. Thus, bloggers who make an endorsement must disclose the material connections they share with the seller of the product or service. "
