There's an interesting service called LaunchRock that you can use as a "landing page" to help your pre-launch site go viral. (see it in action)
It provides a signup page for you that visitors can drop their email into to be notified when you go live.
If you combine this with some word of mouth advertizing, tweet about it on Twitter, share it on Facebook, Google+, LinkedIn (if business related) etc. you should be able to attract some interest.
It is also handy if you have a 1-liner story about your site so users know what they are signing up for: e.g. "The best social TV geolocation sharing site - coming soon"(*)
Also consider providing additional incentive to sign up early - e.g. special treatment/discount offers of some kind if you sign up before date X.
Finally, when you do go live, you'll be able to broadcast your launch to many people - which depending on the social interaction required for your sites success... may be very helpful... if you need that "Critical Mass" to get conversations off the ground.
Even if you don't use the LaunchRock service, I'd recommend rolling your own signup page before launch.
(*) ok, totally made up buzzword fluff but you get the point.
Note: I have no affiliation with LaunchRock (or the sample site using it), but I too am hoping to use the service to launch my own site.