Make sure the fingers you are not using are still curved and resting close to the keys. Don't let them fly up! When you lift your fingers off of a key, only lift them a small distance. The higher your fingers lift, the more distance they have to travel when they have to cover the key again later, which slows down your playing, makes your fingering more inaccurate, and wears your fingers out faster. When you change notes, only move your fingers a short distance.