At the beginning, I'm trying to fill the forum with current, controversial, interesting posts. I persuade my friends to get involved in the discussion. Gradually, I am acquainting well known and distant acquaintances.

As soon as there is at least some minimal discussion on the forum, I want to try promotion through social networks, google advertising, and advertising on my expert and forum-related sites. I also try to arrange a promotion with my partners. On their websites, but also on the facebook pages they manage.

Well i have jump-started one of mine by making accounts and post topics. Then reply with an other account. Not easy to do and keep track of who posts and who replies.

After a few months some real people came and i was able to stop doing that myself.

Also Facebook helped a bit in that case.