In the May 2007 issue of Harvard Business Review, Duncan Watts and Jonah Peretti write

Viral marketing has generated a lot of excitement recently, in part because it seems like the ultimate free lunch: Pick some small number of people to seed your idea, product, or message; get it to go viral; and then watch while it spreads effortlettly to reach millions”

Of course, as they point out, things are not that simple. Most marketers who try to develop viral messages probably fail, and for good reason. In my experience, you need to be smart about viral marketing. First of all, if there is a whiff of pretense, or manipulation, the vehicles carrying your message will drop out and the message ceases to be viral. Second, not all viral messages are sticky. However well you seed, the message itself has to have the ability to spread beyond one or two layers, and continue to remain sticky so that it can continue to spread. As a marketer, one of the first lessons I learnt was that while potential customers gravitate to influencers, if the product/idea does not maintain their interest, they will not spread the word themselves.

So what works? Seeding the idea among people who are likely to understand and buy into it, and then talk about it in public, either to colleagues, peers, or the whole world. Keep them engaged, cause you want them to continue to be the vehicle to spread your message. Find out whom they influence, and then understand the factors that drive interest. It is maintaining and growing that interest from the secondary group that will make your idea successful.