There's some rather good suggestions over in this thread :
http://www.daniweb.com/forums/post724036.html#post724036
For me, personally, it's a question of knowing your members, and being able to see the site through their eyes - and answer their needs. Build it, and they will come....
Focus on content, and orient yourself to quality and not quantity. If you've got the former, the latter will slowly build towards larger numbers.
It an take months ... just don't give up and you'll succeed. The trick is to always respond ASAP anytime someone does come to the site ... even if it's just one person, chat with them on the site every single day. They won't care that the rest of the site is empty as long as they're getting what THEY came for - someone to talk to and help them out and answer their questions. That's what turns lurkers into members who are willing to stick around until the site starts to pick up ... and the type of people who will tell their friends.
Ahhh...the voice of experience. Let me add my thoughts on the subject to those wise words.
1) It will help if you build up some "forum cred" in other forums. That's like "street cred" , only you don't risk jail time trying to achieve it - you just wear out your keyboard every six months. ;) Find a community that seems interesting, sign up, and try to contribute something positive towards it's goals. While you are there enjoying it, start seeing what works and what doesn't work, and see how you can learn from that experience.
If you do that properly, that can help both you and your forum grow. If you are smart, and choose the right sites, you may learn things along the way too. You'll slowly build connections, and those "lurkers" that appreciate your words may just decide to "tune …