The (not so new) trend is to “go viral” with advertising campaigns. I have seen companies planning in great detail how and when a specific campaign shall go viral, and most often, all these detailed plans needed to be revised.
However, there are some factors that actually enable people to share things virally, be it content, links, products or something else. Carla White of Smashing Magazine lists tests, questions and principles of a viral strategy on the example of mobile apps in her article “Key Ingredients To Make Your App Go Viral“. However, I believe the principles work for any kind of viral strategy. Carla’s article is full of sample screenshots, which is particularly useful.