I've been teaching the "thumb" method. Hold the mike at the top in your hand, and strech out your thumb. Place your thumb on your chin. That puts the mike about 1-3" from your mouth, which to me is perfect for a cardioid. However getting them to actually do this habitually is a chore.
I've also considered an "animal conditioning" tactic ... record a sound effect of terrible feedback, tell the actor that if they sing too far from the mike they will get feedback, then play the feedback FX nice and loud whenever you think they're not holding the mike properly. I think most actors are scared of getting too loud on the PA (but that's what you're there for, right?) ... so if you can get them to be scared of being too soft on the PA you may have won the battle. Aside from a few good laughs in the sound booth you will hopefully end up with great sounding vocalists ...
