SEO your forum, add backlinks, generate new posts, and send your sitemaps to google. also setup social marketing rss feeds
I suggest going on youtube and uploading some Justin Beiber videos and placing your link in the video description. That certainly helped my forum grow.
Try to focus on organic traffic more, it is the most powerful thing to get traffic. Organic traffic is when you receive visitors from search engines like google, yahoo, bing and so on. You can try reading some seo guides on getting your website ranked higher. Mainly it is focused on the content of your blog (it should contain keywords that you want to be ranked high for) and a lot of quality backlinks.