Cache Problem

Hiya, I have a question with reguard to Cache At the moment I'm using <form name="blah" method="POST" action=""> but when i go view the page in NN all the data comes up but if say i wanted to print the data it prints a page saying "Data Missing" .. this document resulted from a POST operation and has expired from cache..." and I don't understand why .. because i thought i had this covered using this script .... if ($REQUEST_METHOD=='POST') { header('Expires: ' . gmdate("D, d M Y H:i:s", time()+1000) . ' GMT'); header('Cache-Control: Private'); } could some one please offer some suggestions Thanks Peter