Try google SEO.

put an ad on google

Facebook if you use it is also a good way to get members, just create a fanpage or group and invite people to join the group, and have people link your site on facebook.

Like everyone else is saying, google all the way,

Try to advertise on other forums (Don't act all spam bot like)
Google and SEO is your best bid

I suggest that you hire an SEO Specialist that will handle your website. Simple tip join on networking sites and forum in that way you can drive more traffic to your site.

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xrumer perhaps

bringvisitor.gom and adf.ly both sell cheap traffic.

the more posts, the more likely it is to appear on say google. it will then eventually grow larger

I have a few staff that deal in SEO development for my sites