The same way you get links to any site. You do the link-building process and recognize that you will be doing this for the life of the site. Contribute to forums (which you are doing). Contribute to blogs with do-follow comments (no spamming, not cool). Find do-follow bookmark sites (look down). Publish articles... You know - all the same stuff you read everywhere else. It works. Just get to it.
USfreeads.com is the #1 classified site for getting backlinks. Google also loves it and lot of Usfreeads are there on first page of google. Just optimize it your ad content for the keyword you want to get ranked. Mostly long-tail keywords.
You need to go down the same routes as every other website: > good content/content swaps > link requests > article syndication > PR submission > directory links There's no quick fix - either get on with it or pay someone else to do it for you Take care
Besides linking with classified directories you build links the same way you build them to any site. I like to build one way backlinks on high PR websites and I see results very quickly.