browser back button - "Page has Expired.. " problem

Hi, I wonder if anyone can offer me a few pointers..? I'm building a portal to do various bits and bobs. I'm using sessions to facilitate the user authentication/login, etc. The only problem I have is that when viewing these (.php) pages in IE once the session variable has been set, a "Warning: Page has Expired " page is displayed whenever the 'Back' button is pressed. This means that the user can only access the page again by hitting 'Refresh' (and therefore posting the form data back & executing whatever scripts again). The rest of IE's error message reads, "The page you requested was created using information you submitted in a form. This page is no longer available. As a security precaution, Internet Explorer does not automatically resubmit your information for you." I know from using other web applications (Hotmail is one example), that this doesn't always have to be the case. Is there anyway I can eliminate this security precaution on the *server side* i.e. without telling users to ditch IE ;-) Thanks very much in advance for any advice. - Best regards, Lee