The striking surface should be as if it was coming from either the second or third web spaces (between the index finger and the long finger knuckles, or between the long finger knuckle and the ring finger knuckle). Obviously, you will strike with your knuckle fist and not the web spaces between the knuckles, but if you focus on either of these points then the hand is aligned properly.
The wrist should be perfectly straight to align the bones to transmit force up to and through it. Wrist injuries are a common result of the wrist bending upon impact.
The elbow should be kept close to the body so that the direction of the strike is parallel to the movement of the centreline of the body otherwise the force will stop at your elbow. Correct alignment of the hand ensures that any force which is rebounded from an impact is transmitted up the entire arm and the body as a whole absorbs the force not just the striking portion of the hand.
Developing Maximum Force
The object of the punch is to deliver force to your opponent and NOT to yourself.
You want to deliver as much force to your opponent as you can, in a very short period of time. In addition to striking quickly, leave the punch extended while the body falls into the punch, moving the force through the target. This gives the person delivering the punch a far more effective technique than someone who only impacts the surface.
You want to deliver the force of the punch in as little area as possible to get maximum impact. If you double the speed of a technique using the same push or pressure, the force will be quadrupled rather than doubled. Therefore, speed is a key ingredient.