The problem is that when you view a web page you are most probably viewing a copy cached at your ISP. F5 redownloads that page. CTRL + F5 forces a refresh from the webserver hosting the page.
I don't think there's much you can do other than post a message to say "Hit F5 to make sure you have the latest page"
I get this from time to time on my site - you need to educate the viewers really...
Unless anyone can show me to be wrong (again)!!