The secret is not to talk too much!
Have to agree with that (within reason of course). I do find that most of the inexperienced guys are much more open to feedback AFTER the facts (once I've rescued their botched approach). Exactly how far you let them go is a matter of opinion and circumstances, but I don't think they learn much if you give them a running commentary starting at top of descent.
That said, the recent economics of flight training seem to have brought along some pretty irritating attitudes. Only two days ago, my pre-flight briefing was welcomed with "yeah, all standard stuff mate, good work" by the 500 hour wonder. Sigh.