According to Dan & Chip Heath there are three basic ideas to focus on when creating a viral idea. They are emotion, public service, and triggers. The more people feel emotionally connected with an idea or product the more likely they are to talk about it. The more they talk about it. The more word of mouth you receive from not only them, but the people they tell about it and so on. When people feel strongly about something they tend to think that by telling others about it they are performing a public service. Finally the last key is to remind people how much they care about the product so that it will trigger more discussion about it.
This sounds like a recipe for success to me. When you think about it these 3 things are pretty much the only reasons people talk to each other. If someone feels emotional about something they want to share it. If they feel there is a public service to be done they want to provide it. And last but not least, if they felt emotional or that there was a public service to be done and are reminded of that then they want to again share it.