You can't ever 'create' a viral marketing campaign. All you can do is create a great idea and hope that it goes viral. Many of the most successful viral distributions have occured with almost no planned thought on behalf of their instigators, such as the Clare Werbeloff 'chk chk boom' clip. In other cases, it has just been blind luck, such as this clip from The Amazing Race of a woman taking a watermelon in the face at high speed (although just how many YouTube watchers would have connected it to the show is debatable).
Getting an idea to go viral generally means starting with a truly great idea. Indeed, a lot of the campaigns that are intended for viral distribution are the product of a limited distribution budget and minimal planning.
In this article for SmartCompany I look at one example in detail, the animated short series Beached Az, and spoke to a few folk in the marketing industry about what makes a campaign more likely to go viral.