According to Habitat for Humanity, the average beginner carpenter uses almost 30 strikes to pound a nail when it should take 3-4. Start by holding the hammer at the base and swing with your shoulder and use your forearm, but don’t use your wrist. If your wrist gets tired you’re doing it wrong. Here are some more how-to’s.
Hold a Hammer [Real Simple]
Tips for Better Hammer Use [Hammernet]
Hammer Hints [iAfrica]