A good way to apply Diego's tip is to picture the heavy bag as an actual opponent. The mass of the bag is a good stand-in for the mass of an assailant, which helps your fists and wrists adjust to solid resistance. Mix up your blows in speed, strength, and even locations on the bag. Pick a spot to serve as the head and abdomen to help practice your aim and learn combos. A couple of solid strikes to the kidneys can soften up an opponent just as well as a shot to the nose. Variation is key.