Nails can be incredibly difficult to pull. Everybody knows the old trick of getting a block under the hammer's head to act as a fulcrum. That's still your best bet. But many times they skip that time-honored trick and twist the hammer sideways, using the edge of the hammer head to act as a fulcrum. It's always more efficient to use the wood block and to pivot the hammer in the same direction you swing it. Twisting the hammer sideways is a good way to loosen the handle, or possibly pull it right off.