Usually what I do is by their second or third cross country I'll take out the GPS cards and seal them in an envelope inside the airplane. That way I force them to rely on their dead reckoning and pilotage without the temptation of using that little direct button on the GPS. If they have to use the GPS cards I'll know about it.

As for any advise, keep your eyes outside. Make sure your instructor gives you all the proper endorsements as well.