God knows if they actually work, but I promote for AB, so I'll just give an idea of some of the things I do, personally.
Advertising is not easy work -- Not overnight. Everyone should know that. It's something that takes time, but it's worth it in the long run!
Basically, I advertise, right? I find that the best way to get people involved in your own forum, is for you to get involved in theirs if it's forum advertising you're going to do. Where you really want to focus is on the places that your own forum relates to. e.g. If your forum relates to anime, then try affiliating with some other anime forums.
Affiliation is good. Really good. Get yourself an affiliate button and then get that out there into related forums. People that like what your forum promotes will join you.
Aside from that, you try to get your name out there to different Ad Forums -- Get some packages, do post exchanges (This is a really big deal because when you do post exchanges, whoever does it might like the forum and might join for good, and bring company) You need to participate actively with other forums, though. It's really one of the bigger deals. When you make friends, you make for a better community join.
Another thing I do, is check out A LOT of forums. When you go to an ad forum, go through the ads. Visit some of the places, yourself. More than likely, you'll find a community that you can get to know, and they'll join over. Don't go PM Advertising though!! That's bad. What you wanna do, of course, is put your forum ad in your signature. When people start to get to know you, and think you're cool, they'll click to see where you're from, how'd you crossover, and what you're looking for.
Remember to post a lot when you do that though! Let more people see the signature.
What I found is another way, is through related blogs!! If you read blogs, then go through different blogs, comment on them, and when you do, at the bottom of your comment, leave a signature showing the link to your site. People are bound to click it. (I've done it several different times as well)
A friend of mine also told me about sites like Digg and Stumbleupon that you can use to get some of your topics and threads out there If a lot of people like it when they've found it, then it will keep showing, and more people will see it and get interested!
The real trick to this is really just time. Time and Friendliness. You don't want to advertise just to get members and numbers. You want people for the community, because it's a joined happy community that makes for an active and happy forum.