The hammer is such a common tool, most people probably don't think too much about how to use it. This video from This Old House, however, shows how small differences in your hammer can make a big difference for your work.
The video begins by explaining the difference between several types of hammers that are specialised for certain tasks like metal working and shingles. Later, general contractor Tim Silva demonstrates how to properly swing a hammer. Notably, Silva points out that it's a good idea to use your whole arm with a wide swing so as to avoid straining your wrist.
He also demonstrates how to use straight, side and curved claws to remove nails. As mentioned in the video, curved claws provide considerably more leverage for nails that are very deep into the wood already. However, you can get more leverage with a straight claw by placing a small block of wood underneath the hammer while pulling.