The guys from ninjamarketing.it look like freaks but they do understand something about virality. Their core approach is on the design of viral dna for campaigns.

1. Look at what is (1) enjoyable, (2) valuable and (3) meaningful in the product
2. Identify a psycho-social tension that will leverage the word of mouth effect - catharsis is viral
3. Give a voice to it

The process is : tension > emotion > catharsis