Probably contests and hire some good posters that wont write jibberish. Make forum dofollow and some will post too.
I own a forum (the one in my signature), you can visit there for ideas if you want. Your main focus should be first impression. That will decide whether people sign up or not. You are better having more members than trying to promote the site as the content members write will earn you the place in search engines, plus, they cost nothing for the content they post.