The Wall Street Journal has published a decent look at the elements of viral videos.

They identify three main elements:

• Great content – Many popular online videos are short – under three minutes – and have some elements of cognitive dissonance. Excluding celebrity videos, many popular videos are deliberately bizarre juxtapositions, such as HotForWords – a woman in a low-cut blouse talking about etymology.
• Build a fan base – Charlie Todd, the founder of Improv Everywhere, has staged more than 80 events – such as the No Pants Subway Ride 2009 – and posted most of them on YouTube. “We have a pop-up at the end of the video that says ‘Click here to see all of our other videos and subscribe,'” Mr. Todd says. “That’s one thing that everyone should do on YouTube.”
• Search engine optimization – According to YouTube’s Aaron Zamost, “People don’t know how much work uploaders put into this stuff.” Videos should have clear titles, an accurate description and appropriate keyword tags so that they can appears correctly in a YouTube search. He also suggests using YouTube Insight to figure out which blogs are directing traffic to your video and where your viewers live – so, for instance, if your video is popular in Brazil you can add a tag for Brazil so it will pop up in searches there.

The WSJ article doesn’t reveal any “secrets” – but it’s surprising how many video publishers ignore these basics.