I responded to <a href="http://www.pprune.org/cgibin/ultimatebb.cgi?ubb=get_topic&f=5&t=000509&p=" target="_blank">This Thread</a>, also about balanced flight. Mine, and other's responses have suggestions that should help you.
I would also add that getting your student to follow you through while you demonstrate a perfectly co-ordinated roll <img src="wink.gif" border="0">
is helpful to give them the 'feel' of how much input is appropriate.