Something we do at the game I mod is a weekly memorable quotes post. We post a thread in the OOC every Sunday, and then players can go back and post quotes from threads/journal entries that made some kind of impression on them (made them laugh, could relate to, etc etc). The only rule is that you can't quote yourself. It helps build a sense of community and also lets players feel praised and good about what they write. Plus, it encourages the reading of threads that you might not be involved in.
Another thing we do, (and lately, it's been a bit more sporadic) is a weekly character building exercise. Very similar to the above posts, where we ask a few questions in the OOC and the players respond as their characters.
I would also suggest superlative awards (best thread, most developed character, etc...including fun ones that pertain specifically to your game) after a certain amount of time (6 months, is when we did ours).
We've also done something called a Love Meme (and it's been in a few games that I've been in actually) where you post one thread in the OOC and everyone comments with their journals in a single post, and everyone else responds to that person's comment with love. The catch is that it's done anonymously so you don't know who's commenting. It's fun. The only thing you'd have to do is make sure, as a mod, that everyone has about the same amount of comments, or people might feel left out.