I know there maybe a user selectable setting for this some where, but I suggest reducing the number of topics per page. Some forums are only getting 1 maybe 2 new threads per month but have threads from 8+ months ago still listed pretty high on the page. Perhaps 10-15 threads per page would make the place appear more fresh to visitors?
There's a preference here, David http://adminextra.com/index.php?app=core&module=usercp&tab=forums Number of topics to show for each forum page:
