Okay, I know this sounds crazy, but I did it when learning to breathe properly for flute playing. When you breath from your diaphragm, you don't move your upper chest. Tie a string around your upper chest and practice breathing without moving the string. Also, most people naturally breath from the diaphragm when they first wake up, so pay attention to how you are breathing in the morning. You can also practice by placing your hand just below your belly button and trying to expand that area as you breath.