Put this in a file and include it the first thing you do in every script.
<?php
header( "Expires: Mon, 26 Jul 1997 05:00:00 GMT" );
header( "Last-Modified: " . gmdate("D, d M Y H:i:s") . " GMT" );
header( "Cache-Control: no-cache" );
header( "Cache-Control: post-check=0,pre-check=0" );
header( "Cache-Control: max-age=0" );
header( "Pragma: no-cache" );
?>