One thing I learned when starting out a new website is find other forums that relate to your websites topic, develop a profile, and place your website in your signature.
Make sure your forum is listed / submitted to the search engines (google/yahoo/bing).
Find blogs that relate to your website and make comments of substance and list your website in the comment, thus leaving another link to your site.
Publishing a newsletter that your current visitors can read and remain engaged to can help keep your current traffic.
When used properly Youtube videos and Twitter can be valuable tools in helping to promote your website. (be sure to post videos of interest that people will want to watch.)
I hope these tips help and good luck with your site.
- Andy.
Spend some time on other forums with a similar niche and have a link to your forum in your signature, talk about your forum to friends, there is always someone who knows someone, who knows some.... I think you get the point 😛