that comes with time, focus on snapping the shots, not loading up, relaxing, only clench your fist around 2 to 3 inches before impact, try stay loose
Also, relax the shoulders. I always used to catch myself hunching up and using my shoulders to hold up my hands/gloves, but it's really not necessary. Keep them nice and lose, hop around on your toes, bend your knees, bob and weave a little bit, and when you start punching, snap out the punches when you turn your body and bring them straight up.
