I always used to try and get the student to debrief themselves.
I find this method a bit insulting, after all its the instructors job to critique the student. However getting the students thoughts by subtle debriefing is useful.
The important thing is to get the student leaving the de-briefing on a positive note, wanting to progress rather than being reprimanded, humiliated or criticised unfairly, as I have seen instructors do. Ask yourself, how many times in your lifetime do you remember being praised for doing something and how many times do you remember being criticised. It take a wise selfless man to turn a critical debrief into a positive one but those are the type of instructors that students enjoy flying with the most