There are many factors that you need in order for a video/campaign to go viral. Even if your viral campaign has all of these factors, there is no magic recipe to ensure success - part of it is purely random - you happen to tap into the right market at the right time.
First and foremost, the campaign has to be funny and engaging. This is the only way that you are going to get people to willingly pass along the viral to their friends and get the interaction that you are looking for from your campaign. If there is nothing in it for the consumer they have no reason to do anything with it.
Second, you have to speak with the consumer rather than at the consumer. No one likes getting spoken to, you need to actively participate in the conversation. When people spread virals, they know that it is marketing, but it cannot be self-serving marketing - there has to be something in it for the consumer.
You need to be transparent. The worst thing that you can do is hide your affiliation with the product or what you are selling. If you are not transparent and the campaign goes viral, there is a good chance that it will come back and hit you in the ass. Take a look at some of the viral campaigns that Sony and 7up ran off-brand. When the public found out that they were set up, the negative reaction was far worse than any positive effects the company received from the campaign.
You also need to try and get influencers engaged with the campaign. These bloggers, youtubers, diggers and the like have large networks of friends that are tapped into the markets that you are likely trying to penetrate with your campaign. These are also the people who will crucify you if you are not transparent with your marketing attempts.
And you need to sit back and wait. These things far more often than not do not take off over night. Patience here is a virtue. And keep in mind that if you do not have failure with some of your viral campaigns, it usually means that you are not pushing the envelope enough to have a campaign that will truly go viral. More often than not your campaign will not go viral, but when you have one that does (and youve done it correctly), it will more than make up for the failed attempts. Think about it this way - viral marketing is like shoe shopping - you arent going to find the perfect pair of Manolos for half price every time you go shopping.