this comes care of alien who has done so much for the slackware community. basically, you cron a script i believe he built, i opted to create and use a custom config file, and found it well documented and straightfoward, the script looks for changes in the CHANGELOG.txt and if it finds updates, downloads them and either generates 3 cdrom images, a dvd image, or both.
ethier way it’s a great script mirror-slackware-current.sh and my local slackware mirror can be found at http://drunkensailor.org/slackware/slackware-current/