This article from SitePoint.com pulls together advice from the SitePoint Member Community on how to encourage people to post to forums. The article compares Lurkers to new kids in school, watching how the other kids interact and looking for a place to fit in, and provides five main tips for getting them to jump in.

1) Welcome and help New Members
Have a forum dedicated to introductions and for new members to 'test the waters' with other newbies, without the pressure of posting to the whole community. Respond to newbie posts quickly.

2) Stay In Touch with Members
Sending regular (but not too frequent) updates and bulletins highlighting interesting new posts and topics.

3) Run a Contest
Design it for quality of posts rather than quantity of posts, and provide good prizes that are relevant to the community. Consider getting sponsorship from providers of related products, and give them as prizes. Also consider guest appearances: invite someone respected by the community to post and be available for questions.

4) Use Conversation Starters
Consider setting up a few shill accounts for this purpose, and play the controversy card.

5) Make membership a must-have
Provide benefits to members that you don't provide to everyone. Put the latest posts on the front page of your website, to let visitors know that the community is an important part of the site.