Getting sites to use Expires: headers

I've been pestering the Weather Channel website on and off for months
and months to try to get them to implement Expires: headers in their
radar maps so that Squid doesn't cache them too long. Same goes for
NASA/JPL's Cassini Today page at
<http://www.jpl.nasa.gov/cassini/today/>. And yet, no response.

I even sent specific instructions to NASA on how to implement meta
files in Apache, the web server in use at JPL, to no avail.

I really hate having to put refresh rules for every site that fails to
use Expires: headers. Does anyone have any ideas on how we can
motivate and/or educate these web admins on the benefits of caching
and of using HTTP features that are cache-friendly?

        -Mike Pelletier.

--
 "[It will] be very hard to increase browser share on the merits of
  [Internet Explorer] alone.  It will be more important to leverage
  the OS asset to make people use IE instead of Navigator."
			-- Christian Wildfeuer, a Microsoft Manager