I have no idea what makes a good portrait. Natural light is best, I think. Relaxed atmosphere–yes. Bottom line: I know a good portrait when I see it but not one moment before.
I have a friend who has made a business out of portraiture, but even he goes into every session hoping some magic happens. Capturing the essence of a person is like grabbing for fairy dust. Good luck.
That said, here are some recent portraits that I like.