modrewrite every link so search engines will like it, make it fully opened so search engines can crawl it, make there subforum with actual topics what people are looking for, make there fake posters who will post interesting informations, tell to your friends to bump interesting topics, put top10 topics on homepage, make there contests where people can win something small ... thats all i think that can work ... but im a newbie and those are only my
wish you best with your sites!