I used to be fairly active on several different forums - I was supposed to make a goals thread like last week but never got around to it - however I often see the same few people commenting and posting with a few random members scattered about. The new background was a nice touch (looks a lot nicer imo) but there's always more that can be done.
Servers that I used to be active on would always have the forum achievements, if you can call them that, with the different ranks for amount posted - obviously there'll need to be a minimum amount of characters typed per post as to not allow people to get some nice looking rank on forums just because.
Rewards would also be a good touch to try and improve forum productivity - but not ones that would overly impact the game, perhaps even titles on the forum for a suggestion that makes it into the game - a big one that's actually a draw to the server not like making spawn times of certain monsters quicker. There could also be Forum-based competitions such as signature designs or something random like 'best joke wins X'
Lastly would of course be a dedicated Forum moderator - I've checked the staff list and haven't seen one, unless it hasn't been updated for a while.