If you wanna keep older logs and they are not compressed — like on many Ubuntu 8.04 — you can run this script.
I just call it from cron, every 2nd of the month and save the output in a logfile.
# Compressing all log files in User Web
# Script takes Month as parameter
# like: ./compressLogs.sh 01
for i in `find /var/www/web*/log/20*/$1/*.log`
echo "Filesize: " `du -h $i`
echo "Compressing File..."
echo "New Filesize: " `du -h $newFile`
Adjust it to YOUR needs!