Playing With Profits

Promoting a forum is no different than promoting any other site. Quality link building, content, advertisements, etc. You need to get the word out to as many people as you can, and you need the forum to be optimized for the search engines. Try participating in other forums similar to your niche. Don’t spam but rather participate constructively - you have better chances that your site gets noticed on your forum signature that way.

Attracting traffic to your site is easier than trying able to make those visitors register and participate. That would have to depend on how lively and useful the forum topics are in your forum. A simple way would be to invite friends, colleagues, family first into your forum to jump start the discussions. As conversations build up, a sense of community will also follow and your visitors will come back to your site almost religiously every day.
One of the ways to link building for forum is by submitting your forums to a list of good quality forum directories rather than general web directories. This directories are targeted at forum searchers. Here’s a list of good directories I found.

http://www.forumshowcase.com/
http://www.forumzilla.com/add.php **don’t list forums <100 users
http://www.forumsinfo.com
http://www.foruma-z.com/addurl.php
http://www.bbswatch.com/index.shtml
http://www.bluenoserlinks.com/
http://www.findyourforum.com/
http://www.webmasters-central.com/
http://www.grandeforum.com/

Remember, you will not get a popular forum instantly… It involves hard work and patience.