caching

Recently I changed hosts for my WP blog and subsequently went thru the somewhat complex process of moving the files to the new host and then changing my name servers. ALSO when I moved the site I moved it to a subdomain and not the root of the new host.

The site now works on the new host pretty much as expected.

However I have a problem with caching now that the site is moved.

When I make a new post my system sends out an email to all registered users with a link to the new post. IF the user clicks on that email link they are taken to the site and the new post.

IF HOWEVER the visitor happens to browse to the site URL, the user will not see the new post until they clear their cache and refresh the browser. This only happens on some peoples systems and not on others. This does not happen on any of my systems but happens on both my parents systems. If this does happen on someones system it happens every single time they visit the site, no matter gow many times the cache is cleared.

Any ideas on why this is happening and how do I prevent it from happening?

Doug