We’re all familiar with this arm-toning move, but many of us don’t do it correctly. One of the most common mistakes? Letting your elbows flare out. This not only makes the move less effective; it can also put strain on your joints. Instead, make sure you keep your elbows close to your torso. And, don't let your head drop or your back sag — you want to keep your body in a strong, straight line.