Just to add to louie's solution, you should also take care to prevent possible email header injection which can lead to your form being used as a mailer for spam bots etc...
PHP Code:
function sanitize( $subject ) {
return( str_ireplace(array( "Content-Type:","to:","cc:", "bcc:", "\n", "\r", "%0d", "%0a"), "", $subject) );
}
$subject = sanitize($_POST['select_name']);
$emailSubject = 'Contact Form'.(!empty($subject) ? " ". $subject : "");
if you are using php less than php5, you'll have to use something like preg_replace as str_ireplace is a php5 only function, but lets hope you have php5 and I don't have to explain preg_repalce to you!