put this in the head section. you can also use html, but i just prefer this method... remove the @ if you want, but i would leave it under production mode.
@header('Expires: Mon, 26 Jul 1997 05:00:00 GMT'); // date past
@header('Last-Modified: ' . gmdate('D, d M Y H:i:s') . ' GMT'); // modified
@header('Cache-Control: no-cache, must-revalidate'); // http 1.1
@header('Pragma: no-cache'); // http 1.0