You'll notice the face of the hammer head striking surface is very slightly convex. This is to allow this next tip.
Time your hammer strikes so that the last blow will set the nail head flush with the surface of the material your nailing. This requires countersinking the nail head ever so slightly so the thickness of the nail head is into the surface. What you want to make sure happens is that the nail head is not sticking up and that you make sure the nail is "set" properly and is flush.
