Document expired on back button after form submit

Am having problems with webpages expiring.

Have a php login form which uses POST.
On a failed login user lands back on login page with option to open a new page to contact us. If they open that new page, then hit browser back button, the login form has expired. (IE -The local copy of this webpage is out of date, and the website requires that you download it again, FF - The requested document is not available in Firefox’s cache)

Can anyone advise the best way to handle this?