My first forums I ever owned turned 2 years the other week, I now have 100 000 members and over half a million posts. I had advertised my site on a couple of sites that I was active on, people who knew me on those sites wanted to help me get started, so they put my site as their nick name on msn, I then had a few hundred in the first few weeks from that and word of mouth. It got things started, then google took over after that through careful seo and content.
Good luck!

IT took us 6 years to get to one million posts (nothing much to boast about) however since June 07 till today, I have received a further 119,000 posts :)

I started my forum in February 2007 and I have now managed to reach 11,000 posts and just over 200 members and a PR 2 on google in that time, it can be time consuming and can take alot of time to get a forum up and running, basically what I did was spread the word around to friends, family and using my MSN name, I also went on alot of Link Directories and submitted my site to those 2 that I can recommend are

Linkalizer
Link 2 Me

I also spent quite a bit of time on forums similar to my own posting on them each day with my link to my forum in my signature.

Also submitting your site to search engines like Google, Ask, Yahoo can be a good help aswell.

1,708 Members in 5 Months

Sponsored ads on Yahoo and Google , Link Swaps , Contests and Give Aways Ect.....

I have been using Chat-Bots in the Battle.Net for Warcraft 3 Players. I got quite many and then I had the "Limited Guest Viewing". My board exploded, first I was using SMF, I got 15 000 Users in 2 Months. As I knew I could not keep using the Chat-Bots I started learnong about Search-Engine-Optimization. I added my website to linkdirectories (www.submiturl123.com) and kept reading and doing. I get about 64 % of all my visitors through Google now. Members are not forced to register anymore that's why I do not get that many members daily anymore. Members usually only register now when they have questions and want to share there Hacks or Cheats, as my Website is about Cheating and Hacking Blizzard Games (World of Warcraft, Starcraft, Warcraft 3, Diablo 2), or simply to disable Advertisments as they are not shown once you are registered.

My board started in February, should be the 16th February and now has 26,973 Members. I get about 10 $ per day with Google-ADsense. 2 Euro a day for my Layer-Advertisments. The "big-money" is not in those Advertising programs but in real-partners who directly pay you. My suggestion to you, try to get into touch with Search-Engine Optimization. I outplayed several other boards similar to my as I kept doing SEO.

Thanks for your ideas!