Most times it's just word of mouth. A lot of forums, if not ALL of forums I've ever joined are because someone told me. Put the link to your forum in your sig of any forum you belong to and if you know of any friends who happen to like video games, let them know about it.
Also, if you have AIM or any other messenger service or any other site where you have a profile or anything, include a link there. Try to get it out as much as possible. Be warned though, that as much as you want to get it out there, don't force it down someone's throat either. If it's a good site, people that belong to your site will start sharing it to their friends without even thinking.
I hope this helped!
"I would feel more optimistic about a bright future for man if he spent less time proving
that he can outwit Nature and more time tasting her sweetness and respecting her seniority."
E. B. White (1899 - 1985)