Well, lots of forums that I go on have a restriction on viewing threads if you are not a registred member, e.g. if you were not registered on Habbox Forum, and you tried to view more than 3 threads, then a page would come up saying "You have reached your weekly supply of threads, this is because you aren't registred at Habbox Forum! Click here to register to see the thread you were trying to access".
This would surely help this problem:
Currently Active Users: 164 (81 members and 83 guests)
83 GUESTS? that is more than the members we have online! even if these 'guests' are registered, then it will encourage them to log in, and become more active.
Just a thought, sorry if this is already installed... delete this thread if it is. (have been registered for quite a while, so I can't really remember back to when I was new and was viewing threads as an unregistered user)
Plus, if it is already installed, make it a bit more stricter, so that it encourages people to register... but not too strict.