Flyers, going along to other twingo meets and letting them know about your site, advertisement (ie signature link) on all forums you go on (Some forum owners don't like this though) and basically just trying to get your name out.
Best way to do it is to offer features that your main "competitor" doesn't have, so people make the switch. Otherwise there's no incentive to use your forum, instead of the one they are currently using.
Go along to all the shows in the summer too, maybe get some jumpers or coats done with your logo/url on the back.
Whats the url?