In reply to mikekeswick:
Two solutions.
Use a 'shit sandwich'. Basically, say something good, ie I always like to see it when you've used the rope as part of the belay. Then give the critisism, ie perhaps you should have looked a bit further back, that thread would have been bomber, next time try going for something a bit better, It'll give me a warm feeling while I'm climbing. Then finish on something positive, ie great belaying by the way, just the right amount of slack for me, just how I like it.
So, nice bit of bread, shit filling, nice bit of bread. He/she probably won't even notice the negative, but will take it on board. Keep doing it to reinforce what you are after.
Or.
Whilst taking a break (lunch, or while waiting to get on a route) at the bottom of the crag. Suggest a belay building activity, where you rate each others (ie 4/10, 9/10), sell it as exchange of knowledge, and how it is never too late to learn something new. Then give the reason that you think it deserves the score you give it. The benefit of this is that you can reasonably give honest feedback in a safe situation. And secondly, when he talks avbout your belay, you can get across the reasons why you do it the way you do, and the safety benefits of it.
Of course, we'd like feedback of how it went with him/her.