First, promoting forums on forums isn't a good idea (and actually it's against most of forums' TOS).
The only thing you are allowed to do on other forums is to put a link pointing to your own in your signature. Can't say that it will drive tons of traffic, but it's a good way to attract more attention.
What will really help you to promote your forum is:
Uniqueness and hot discussions. The Web is crowded with forums and communities on all sorts of topics. But if you manage to stand out, to spark discussions that are really important and aren't discussed anywhere else, you will definitely get more visitors.
Blog. That's right. Sure, blogs and forums are two separate things, but an outstanding blog post will often attract attention to content that your forum might have. So you can create a great article, share it on the Web, and when people check it out, you may direct them to your forum.
For example, you can add a link at the end of your post saying "now you can discuss this issue here", etc.
Social media. That's a great channel to attract new visitors. It's ok if your first attempt to use social media sites for your forum promotion wasn't successful. Make more attempts, be more creative, and you will see it will work.
Invitation of experts to participate. The influential guys can get some free advertising on your forum and perhaps a chance to promote their product, and you'll have the benefit of bringing more experts into your community.
Organizing contents. People like to have fun.
Opening Marketplace. That may potentially drive more people to your forum (but it won't work if you have too little community of forum members).
Summing up, promoting a forum isn't easy. And some of the tips I gave you aren't easy to implement. But if you try, you will see that they will bring great results!
