The Performancing guys talk about their approach to promoting their site:
Pinpoint the audience and start writing for them.
Do all the normal things you do to promote a blog, including a little strategic advertising to kick start if you can.
Develop the community, put them above all else.
Invite key members of the community into alpha’s, beta’s etc. Develop a sense of ownership before launching
Launch and let your community be the first evangelists for your product