Avoiding 'page expired' warnings

Hi I've got a PHP powered site that involves some form submissions for searches and when using the browser's 'back' button to return to the search results page I get the horrible 'Page expired' warning due to the page being created from variables sent in a form. I know there is a way to avoid this problem because all search enabled websites seem to manage to avoid it, but I don't know how. Can anyone help? I had thought using sessions would help but it makes no difference. Is this something that can be avoided with PHP or does it require some client-side stuff? Thanks Scott Graham, on behalf of Brian