reciving warnings on browsing backward from php pages

Hi all i am using sessions on each page of my php to make sure user is loged in but when press browser back page i get the following error and once i refresh i can see the page. It is very annoying to see that every time and i know the session did not expire.Could any one tell me how to solve this problem and not to recive such warnings.Thanks

Code:

Warning: Page has Expired 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. To resubmit your information and view this Web page, click the Refresh button.