To give your subjects a sharp jawline, have them shift (not tilt) their head directly toward the camera. This stretches the skin under the jaw just enough to sharpen the jawline.
Taking this tip to the next step, position your subject so that they are looking up toward the camera. Usually, you do not want to shoot upward toward a face, but if you are above the subject the angle remains good. Ideal is for the subject to lay down, which makes the tilted-back neck natural. And the tilted-back neck will stretch the skin below the jawline.
Camera and Lens Settings
135mm f/2.0 1/200s
3840 x 5760px