1. Seed with high quality community members. Initially this might just be a small circle of friends or emigres from another online community.
2. Be really hands on with identifying, engaging with, and empowering the best community members. Typically, this would mean giving them some sort of moderation powers and/or giving them access to some sort of "backchannel" (ie, a mods-only chat or forum) in which they can be a part of the discussions where you discuss community direction.
In general, treat your small (initial) size as an asset. Your community cannot represent a Stack Overflow-sized massive knowledge base.
So, what can you offer that a large existing community cannot? Chiefly, this would be an ability for members to get in on the ground floor and shape the direction of the community while having a direct line of communication to the founder(s).
Hands-on communication with a community founder can really reach people. Think about what it would mean for Paul Graham to reach out to you personally in response to an HN comment you made. In the early stages of your community, you can be the founder and make those connections.