One of the best tips I ever got was from my table tennis coach. He watched me play and said, “You are leaning forward with the wrong foot!” I’d been playing for years and thought I had a good stance. I’m left-handed, and no one had told me that I should put my right foot forward. I had copied the stance of other players without noticing the problem. It was obvious once he pointed it out. But until then, I hadn’t thought to question it, and more practice hadn’t helped me notice. That single observation improved my game more than...