Learn to do a 1-2 palm-heel strike, and how to use the hand effectively. A palm strike hits with the bottom part of the palm, where the hand meets the wrist. The hand is held perpendicular to the wrist to avoid hitting the softer inner wrist tissue against the target.
The bottom ridge of the palm is a surprisingly solid striking surface, and can do just as much damage as a closed fist when utilized properly (some studies have shown that a palm strike actually can produce more energy then a a punch in most cases), with far less risk of injury to the striker's own hand.
The palm strike is useful as it is thrown in a more relaxed manner than a clenched fist. This is because clenching the fist shortens the extensor muscles of the wrist which counter the action of flexor muscles of the wrist used in punching.