I think the session storing will work, but you must also remember to session_unregister() the variable, or the form will be worthless after the first submit. For security, GET/POST cannot change a registered varible.
I think the best way to do this is to session_register the variable after the form is submitted. Then, display the form again with the variables instered and session_unregister the vars again before so the form will post new values.
I was hoping to find a way to make the server globally store all the form post info or just not "expire" the page. If anyone knows how to do this on Apache, please let me know.