If you have a section where people can link to their site / blog and then invite discussion etc that might help a bit - lots of people will post often just to get traffic to their site which is quite acceptable and rarely spammy.
Is there an 'off topic' area? That can keep people busy too. Been in a fair few forums on differing subjects, and have seen that work. There was even one I saw where in the 'off topic' bit there was ONE politics thread and ONE religion thread. Kick it off and let everyone get on with it! Call it 'The Dark Room'!!!
Geddit?
I'll shut up now.
