A number of viral techniques have been suggested to increase the membership of my website.
If your website / application has user generated content, build in a feature to syndicate that content to social media platforms: Twitter, Facebook, etc.
I think sharing or publishing content, that contains links back to your site, in a social context is a big win.
Word of mouth (the old term for "viral", I believe) always works best when your members can easily communicate the benefits your that site has to offer to their contacts. How it is delivered is secondary.
On my site, a large group of users are actors. They can create a "Talent profile" and submit it to different casting opportunities, frequented by other talent.
Additionally Talent and Crew members can use a custom button linked to their profiles on my site, similarly to the one below.
We've been using AddThis and have been pleased with a few things.
First - it's user controlled - it's up to the "sharer" how they choose to share.
Second - there are lots of options - everything from Facebook To Twitter to Digg to StumbleUpon, etc.
Third, it includes an e-mail option. Simple, user friendly and not intrusive.
In addition, there are some great analytical tools so that you can easily see which tools are being used the most on which content. You can't do that if you're just asking / hoping that people will use their own e-mail systems or post on their own to Facebook.
Last, people seem increasingly comfortable in using this.
Net - I'd vote for using AddThis.
If your target audience are young males - run a calendar girl competition. Done right, it would blow up your traffic.
My favorite viral technique is to try, try, try, try and try again. For your audience, your product...it could be any variety of things that will work. So keep trying.