Word of mouth, telling your real life friends, and sig ads. There's other ways, like sticking papers with the domain in similar topic books in a public library or two.
If your site is about anime (a non-download or streaming site), you could sponser anime conventions. You get an ad in the booklet that is given to everyone. Basically, look into the niche the forum or site is about and find ways to reach people.
Take a look at: http://www.profitbaron.com/2007/12/05/20-tips-how-to-improve-your-alexa-rank/ Mentioned on the forums a while ago.
You can have you forum link in your signature, be a good community member, word of mouth. Do link exchanges with other websites/forums of a similar niche.. post exchanges can also help. You can get further help here : http://www.adminfuel.com/
Forums are best populated by people that understand forums. You find those people at existing forums. Best free advertising...? I guess signature links. There are other methods of course but that's one of the simplest and it's free.