Punch properly- that is, by shifting your weight on your feet twisting your hips and shoulders. Your punches, especially the straight punches, should come directly from the shoulder and be driven be the sudden turn at the waist/hips. Timing is also important; it doesn't make much sense to be so fast that you are out of stnc with somebody you are trying to counterpunch. Timing is more important than speed. Understanding range and distance so as not to be over extended and reaching with punches. On the double end bag try this; start it with your jab (always fully extending the arm) and circling. With every other punch only hit the bag as it comes to you and move your hands as little as possible- envision hitting it with your shoulders.