Finding and culling unused entries from zone files

[Keith Christian]

I'm looking for a method to cull out unused DNS entries from zone files.

Eventually, entries we add to zone files entries fall into disuse.
Usually the customers never report when this happens, so the zone
files grow and grow and grow with "cruft."

Can any of you share recommendations?  I'm running bind-9.5.1, are
there logging entries that are particularly useful for statistical
collection that could be analyzed to offer guidance?

Thanks.

=======Keith