For an excellent article on how to leverage your customer base for an effective viral campaign, read the Fast Company article Amateur Hour, Web Style.
It highlights that the key ingredient in every successful viral campaign is the recognition from marketers that for an effective campaign, consumers will be wanting to advance their own interest, be it showcasing their video production skills, acting skills, or some other unique skills that they wish to put on display. Case in point being the Diet Coke and Mentos guys.
So for marketers wanting to effectively use User Generated Content co-creation viral campaigns, they must be willing to understand, motivate and reward consumers narcissistic behaviour.
After all the word Amateur is French for "lover of." Defined in Wikipedia as: The word comes from French, and can be translated as "lover of", reflecting the amateur's motivation to work as a result of a love or passion for a particular activity.