1. Formulate your marketing message. Think about product or service you are advertising and create a message that communicates the benefits and uses of what you are selling.
2. Launch a website advertising your product or service. Your customers will be able to direct their friends and so on by sending them to your web address.
3. Make the content on your website sharable. For example, you can allow readers to embed a funny video from your website onto their own blogs. An “email this article to your friend” link is another way information spreads from one person to another.
4. Use email as a viral marketing tool. Include a marketing message about your product or service in the tag lines of your emails and also include your advertising message in auto responder emails you send to those who email you.
5. Post your content on other Internet sites such as message boards and blogs. However, do this carefully; many forum managers are now aware of this practice and may delete your messages if they think it’s spam.
6. Incorporate your marketing message into rich media. Video clips and Flash games are very popular on the Internet, so if you have a great idea for this medium your message will spread like wildfire.
7. Spread your message off line as well. Tell your friends and colleagues about your product or service and hand them business cards with the URL on them along with a catchy tag line, perhaps the same one you use in your emails.

Viral videos are often humorous in nature.  Here are several examples of Viral Videos:

David after Dentist
SNL Lazy Sunday
SNL Dick in a Box
Star Wars Kid
The Evolution of Dance