After I injured my back in university, I ended up coaching our college team in a university touch-tackle tournament. I learned you can use your brain to outsmart brawn and that a coach only succeeds when his or her players perform. Coaching my peers was an invaluable learning lesson.
As a parent I coached basketball and some soccer and baseball. I liked coaching basketball most because you were right on the floor in the game influencing the players. The key to success is to focus coaching on the defense, since all players practice shooting. While I loved baseball, coaching was mostly pre-game. And with my dearth of knowledge, soccer was more about stressing continued effort and to use both feet in kicking. Plus I yelled a lot.