Like any website, add good content to your forum and the users will come. Update it daily to give them a reason to come back. Make sure the forum is SEO'd and promote it any way you can.
Try writing a few articles on the forum's subject with questions in it, this may get people to sign up to post a reply.
Get some people to fill it up with content first before advertising it. People wouldn't want to join empty forums, but if there are already interesting posts there, then they'll be interested to join.
sometimes ive seen people do the post exchanges and after they have done their posts you get a couple of members who post once and dissapear because there isnt anymore new content. Do you suggest doing lots of these post exchanges till the forum is lively so members dont leave or what?
Introduce a referral program with the help of existing members, offer incentives for a particular number of postings in your forum like giving them a trial product, offering a little free advertising, giving them a link on the main page, allowing them to post their website in the main forum, etc.
You can offer a free domain name for a year, start a reward store in the forum where in order to purchase store items members need points and they must post to receive points.
Everybody does contests. They work, yes, but most of them are short-term...or temprorary. They work while you are holding the actual contest, but after that you're back to where you started (for the most part)
Try to offer something unique...Give your members a reason to KEEP coming back AFTER the contest...not just coming back during the contest, for the prize alone.
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