Viral marketing is one of the most difficult internet marketing techniques to do because there are no guaranteed results. That's not to say it can't be done.
It helps if you have a following, and you have people who are willing to promote your content for you. Social sharing buttons make it a lot easier these days, so make sure you have all the most popular services on your website.
There is different types of content you may share: photos, ebooks, and software. Photos are quite popular on sites like Pinterest and Facebook, so you may want to consider uploading your photos to these types of sites. Pinterest is rather special because it ranks very well in the search engines, but it also does well internally, and if you have enough of the right following, others will share your photos for you.
The most common type of viral marketing is with ebooks and software. Usually it is done with something that is free. If you can make it brandable so that others can embed their links, then it's better because they are more likely to share.
Before you start your campaign, you'll want to do a little bit of research. Browse the most popular forums on your topic. You may also use a site like Facebook or other social site to help you find content as Facebook has lots of groups and pages, and this will allow you to find out what people are talking about in your niche.
What you want to find out is what they are talking about. If you know what others in your niche are talking about, then you'll know what to write about or what type of tool others will need in order to get the job done. This is your starting point for creating your resource.
You then want to create your resource. It helps if you have an affiliate program. Having an affiliate program will help you have a better chance of giving your resource away because your affiliates, as well as your readers and customers, are more likely to give your resource away. That's not to say they won't give it away anyway. It's just a good idea to give them a good reason.
Add your resource to the top software directories, and if it's an ebook, to the ebook directories. Promote your resource anywhere you can as long as the site offers targeted traffic. You can use it to build a list, but it is more likely to go viral if you give it away outright.