How to keep a yum repository up to date?

Hi,

I have a question about yum repositories.

I know how to register a server with RHN
I know how to set up a Yum repository
I know how to change my yum config so it points to the new yum repository
instead of RHN.

BUT

My yum repository was created by copying all the RPMs off my install media
so it is a point in time effectively.
How do I keep that yum repository up to date?  How can I ensure those RPMs
are always the latest ones?

Would yumdownloader or yum-downloadonly be what I need - if so how can I
set that up (in cron) to refresh my repo?

GXW