If your host supports mod_headers, add the following to your .htaccess:

<Files *.html>
Header set Expires "Mon, 26 Jul 1997 05:00:00 GMT"
Header set Pragma "no-cache"
Header set Cache-Control "max-age=0; must-revalidate; no-cache"
Header unset Vary
</Files>

If this causes 500s, then your host doesn't have it installed. If that's the case, hopefully your host will be willing to install mod_headers, otherwise there isn't much you can do.