Beagle - 7:08 pm on Apr 12, 2006 (gmt 0) --Also, as was said, push what's unique about your site that others don't offer.
Building on FourDegreez idea - If you have a sponsor, can you offer special deals to "charter members" who sign up while the site is still getting off the ground? Be upfront about it being new, but excited about what you want to offer, so they feel invested in the project and will want to actively help it grow. If your business model doesn't include charging for the service (hence, no "special deals" to offer), there might be some other perks you could come up with for charter members.
--Also, as was said, push what's unique about your site that others don't offer.