A few more ideas ..
Do you have a newsletter?
Tie the forum into the newsletter ... and vice versa.
Show recent post topics of interest in the newsletter, and ask your subs to participate ... and archive your newsletters in the forum.
Tie both into your blog (assuming you have one).
List your forum in the Forum directories ... and if you can, ping the RSS dirs. with your RSS feed.
Write and submit forum topic related articles ... submit ... archive.
Participate in as many related topic forums as time allows, and list the URL in your sig. file.
Don't force posts to try and draw interest ... regulars will pick that up, and you'll be scorned ... be natural, helpful, and network with others.
Marketing is all about relationship building ... forum building is no different.
Quality and exposure go hand in hand.
Regards,
Mike "don't forget to optimize for the SEs, too" Merz