Hmm one day I'll write a manual on this. Here is the short short version:
A) Start with an existing group of people. Nobody likes to be the first one to show up at a party.
B) Get your ducks in order. Establish the rules and moderators before you go live. You don;t want to learn on the fly.
C) Make the site searchable. Able2Know's growth is almost entirely due to search engine optimization. Almost all forum software has unsearchable session IDs. Find a way to make them work with spiders.
D) Make registration easy. Forum software signup pages are ridiculously unuseable. This is because most forum software has horrible "usability" they are made with advanced web users in mind.
E) Skip the banners. I have used banners in tests and got 0 results. I don't advertise this site much as SEO gets me more members than ads but you might remember me saying that I don't get any return from banners. If you want to advertise use PPC
ads. Contextual ads get members. Banners don't. Make sure your ad is for forums. Don't advertise wigets, advertise "wiget forum". Forums don't work like most sites. Internet users for the most part ignore forums. You need to target the ones who are interested because the majority of surfers are not interactive users.
F) SEO SEO SEO SEO! I get millions of hits from seach engines here. That is where all the new people come from.
G) SEO! he he, i do stress this.
H) A referral program. Word of mouth is the second most important source.
BTW, all the 404 pages go right to the sign up form but this is only temporary as I eliminated an old portal that had a few hundred thousand links in various search engines. When i tool them down i figured I'd send all the 404 traffic to the signup pages as the user might think registration is required to view the content he/she was searching for. This trick has netted a few hundred visitors (I guess, about 50 since I started tracking) but it only works if you have intentional 404 traffic.