Here is a way I did it for a surfer forum I had.
1) Create a free section for anyone to see
2) Create a Members only section
3) Create 4-20 usernames and have them all post (have conversations with your other usernames)
4) Have those 4-20 usernames post outragous amounts of content
5) Post a whole bunch of stories/erotic tales (good for SEO)
6) Buy traffic from link Choker or Ravio
7) Watch the signups
Its a pretty simple forumla.
Jim