Mental ones, not the icky kind (you pervs). Thomas Baekdal has put together a nice list of Seven Tricks for Viral Marketing and illustrates them with some successful viral videos (man attacks bear!) Some of his observations: make people feel something — positive or negative. Do something unexpected (humans love the unpredictable). Standard advertisements will not catch on (believe me, I’ve had some fall very very painfully flat). Never restrict access, and make the content easy to share (should be a no-brainer, since the slightest speedbumps can really slow viral spread, but you’d be surprised at how many campaigns put obstacles in viewers’ paths).