in the appropriate virtualhost in httpd.conf to set the necessary configuration parameters. But your customers may be looking for something a little more under their control.
The next best thing would be using a revision control system (darcs, svn, git, or some other--it doesn't really matter which, so long as it can be installed on your client's servers). If you aren't using one already, you should be!
An RCS means you can, much like Anyango suggested, have a cron job which does 'svn update' or 'darcs pull' or 'appropriate-command-for-your-rcs'. Unlike blindly downloading updated files, use of an RCS is much less likely to clobber your users' local changes.
But that might not work, if your users are on normal shared-hosting accounts. In which case, the next option would be rsync. While it would clobber any of your users' local changes, it only downloads changes to files so tends towards being pretty light on bandwidth for updates.
If that doesn't work, tell your users to get better hosting accounts. :P