I agree with Mark: Post helpful and encouraging comment in forums and discussion groups, and people will discover your site. It takes a lot of time to build up repeat visitors, but it's rewarding to see numbers climb from week to week. Thank you e-mails are even better!
I'll also add that content is very important. You can spend a lot of time in forums, with
SEO, etc., but if people don't find a lot of content, or a lot of useful content, they won't come back.