Also, you can investigate yum-pull (check Google).
The problem with just using the yum cache is that it will only keep the packages that your box has installed. If you are trying to mirror the repositories so that any machine on the network has access to any package including updates, I think yum-pull is a better bet. You can still restrict classes of packages by regex that you don't want to mirror at all. Until you get your regex's established, you may download packages that nobody needs, but at least they only come down once. Then you can simply switch all the boxes repos to point to the ftp server you set up with your own repositories.