I've never done this, but my strategies would include: Create a landing page that ranks decently and drive serp hits to it, Make sure that the user knows how to use the system before they sign up (demo video), if you've got the money, run an adwords campaign for your target demographic, and try to make sure that your existing users invite others. Outside of that, friends, family, yayhooray, here, digg, put it EVERYWHERE. You can always limit the number of beta users if it's out too many places, and it'll help your ranking later if there's loads of backlinks.
I have tried conning my friends, as well as getting Mechanical Turk workers, or posting on various sites like here, twitter, etc. Usually less than I hope for, but more than zero!
