Share it in all forum areas and social networking places and introduce your forum and invite them to join.

If you will provide do follow links in forum it will help lot and people will participate more.

Engaging on niche forum is one way to promoting it. You can participate on other techniques as well like social bookmarking and online group posting.

Initiate some interesting topics.

Make your forum more interesting then you can expect more visit.

It is also a good idea to inform forumers about your forum, and this is actually one of the best ways to get members to sign up and start posting. After all, forumers normally enjoy the forum routine.

Through link building you can promote your forum site.