yeh just don't post about it too much cause you don't want to irritate people. also, people don't want to sign up to a forum that doesn't already have a reasonable amount of users, so if you got any friends that would be interested tell them. ( word spreads easily)
unfortunatly all my friends seem to be pretty anti forum and internet, so it is harder for me.
also I assume you've submitted to a search engine like google. if not you can submit your site to google here: