Echoing Zack, I'd say target specific users if you can -- influential individuals you might write about you, share your content or otherwise support you when you launch.

To catch their attention, though, you have to first figure out *who* they are -- and drilling down like this can be challenging and time-consuming at the beginning. It's worth it to have that bang at launch!

One other tip -- consider guest posting on blogs that cater to your target market, and send readers back to an opt-in. Piggy-backing off other brands' already-established communities is often a quicker and smarter way than trying to grow your own from zero.