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Forum Moderators: rogerd
Expanding features and adding other things that the regulars can participate in is also important. For example adding a mediawiki.
Prune your membership of the users with 0 posts who've been inactive for more than a month or two. Then delete orphan threads older than 7 days. Combine your related but lightly used forums together. Add a great new style. Then email your membership advising them of some cool new changes.
One thing I did to stimulate discussion was to create a "News scavenger" forum which has new threads created based on news feeds from popular social sites. Once the thread receives a certain number of replies, it automatically gets moved to a general discussion forum.
I'd focus on classic community building techniques: posting provocative topics, keeping the discussions flowing, welcoming new members, and ensuring that nobody is made to feel unwelcome. It's slow, but effective.
Prune your membership of the users with 0 posts who've been inactive for more than a month or two
I think that is a little drastic, I've many members that only come around occasionally with a 0 post count. My guess is they sign up for the forum features such as auto reply. I've been pruning about once a year, inactive for 2 years with a 0 post count.
If you have a lot of readers but not many posters, check out this thread: Pulling Forum Lurkers out of Hiding [webmasterworld.com].