Maybe a way to rate useful posts (either useful news or helping people out etc) and the better rated your post is, you can get points that way? That would discourage spamming but still encourage forum use-age. It'd also encourage people to be helpful (though this is already one of the better forums I've ever been on for having a helpful community)
While it would work fine in theory it would be hard to implement. Users would have to rate hundreds of posts per day, people would tell their friends to rate their post highest etc. Not to mention that it would be a somewhat difficult forum mod to write.
