Establishing a forum takes a lot of time. I established a community back in June - and now, half a year later, I got 160 members and 6000+ posts - but I'm quite pleased with that. It's the first 100 that are very hard to get, especially when there are dozens of forums out there with similar content. And in the beginning, a lot of people 'fall off' as the forums aren't active enough, and there just ain't enough posts.
As said above, you have to be really passionate about your forum. Make the members feel that you love it and that you feel pride in it. Work hard, post a lot, answer everyone, send PM's to people. Control the discussions and ask loads of questions. Encourage people to spread the word. Try to pick out a few active members, and promote them as moderators. Make them feel like they have a kind of ownership in your forum.
Then yeah, participate a lot in other forums. Don't spam them, but people read signatures. If you post well, people like to check out your signature. Most of my members come to me as they've seen me in other forums - and once they come to my forum, they love it so they stay ;)
I believe it's important to be unique. Because there are loads of forums out there, therefore, I had modified the forum in every possible way, added useful functionality and used a unique design.
There's so many issues, and there's not a simple answer.
You can see what I've done, at
http://www.mindsofintelligence.com/forums
Good luck mate.