Everybody share your thoughts and ideas to attract your forum visitors in becoming active contributing members.
Content always attracts any kind of visitors in any niche. So build attractive and useful content, start discussions that create incentive to join and participate in them, build monthly or weekly contests to show your visitors you have a healthy & fun loving community etc.
Create any kind of contest if the members completes a task then they will win a prize. Its not much but that I would do.
Create incentive discussion that will encourage visitors come, join and contribute.
Usually visitors are more likely to great features of the website and the beneficial factor that it can provide for the users.
Always keep active on any discussion in the forum. Take part in contests and share thoughts and content as per the forum discussion.
The main thing is keeping the forum active. An highly active forum always attracts visitors and tend to make them members.