The best of the word of mouth campaigns that I’ve seen, and the few opportunities I’ve had to be part of a promotion, are executed by using strong rules for disclosure, plus the opportunity for the blogger to take the product or service and do with it as they will (versus sticking very tightly to a message). By allowing the blogger to work within their community and communicate in a way that’s appropriate to the people around them, the campaign’s intent was still spread, but without any kind of degradation of trust or relationships
Chris Brogan