Make sure your log rotation scripts are configured and working
properly.  If you’ve disabled crond for some reason it’s likely
that none of the log files in /var/log are being compressed and
periodically deleted.  Another commonly filled directory is
/var/spool/mail.  If you seldom or never check mail on your server
this could consume considerable amounts of space.  Make sure you
either forward that mail to an address you’ll read or read it from the
server directly.  In either case you will have to delete files
before any compression can take place.  Be aware that large files
may not fit into temporary space so keeping /var tidy can be critically
important.