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.