One of the things I've had the most trouble with was modeling the human ear. In previous attempts what I got was sort of a vague ear-shaped blob, so for practice I decided to find a reference photo and do some practice with ear modeling. First on the head model with ZAdd on, I would draw a slightly curved line a couple of times where the ear would go and then I would go into edit/move and pull the ears back. Then I went into ZSub and made sort of a large bowl-shaped depression in the ear and then I turned on ZAdd and added some volume around the rim of the ear. I added the rest of the details using combinations of ZAdd, ZSub and edit/move.