Take a look at popular forums and try to work out why.
My advice is PASSION. ignite the passion of people and you will have a busy forum. Keep it clean, be strict on spam posts, send private emails to new members thanking them for joinging. By private I mean if they put a site in the link visit it and mention it in the email.
Make them feel at home
I do it with my rugby forum and it seems to be working ok. To bolster visits, email out now and again a quick email thanking them for being a member at the forum, and list a couple of good threads with links so they can visit from the email.
My rugby forum past 1/3 of a million posts this morning