Other than promote, I make sure my site is plastered somewhere in my profile, on my signature, or through the content I type. Always make sure to find some sort of back-linking.
I try to promote as much as i can. Then i get on posting packages, exchanges, and being a post monster on my forum so people have a place to post. The best thing you can do is post your site on a promotion site, no matter how big, because you'll find the more you advertise the more members you get
Everything mentionned above ... Backlinks and promoting on other forums are great ways but , I also noticed that the more active you are on a site , the more the community gets to know you and the more they will come check out your forum.
It's a frustrating process sometimes. Some days you will have 3 ,4 , 5 members joinning and others zero. Just keep at it and promote!!!!
Advertising everywhere I can. Although most of my members already knew me from another forum, and I either invited them in the chat or they saw the link in my signature. Exchanges don't really work in my opininion if you want them to be active. Unless your doing an active for active exchange. But it helps if you just want to get the member count up.
The best way, well I don't have much of an answer to that, simply go on a lot of promotion forums, submit your site to many search engines, tell your friends and have them tell their friends and so on. Not sure what else.