In my days I have ran many forums and I know it can be a pain in the back side when it comes to managing as you dont get the time to check through every post and reply made.

So on one of my previous web sites which became quite popular i took the time to archive sections that were 'x' ammount of time old and archived it on to a sub domain with an index for convenience. I know it takes quite a while to setup but once the ball starts rolling it becomes easier to manage everything in general.

Just an idea...