The easiest way to avoid getting hit in the ring is to use your feet to get out of your opponent's range. An agile athlete who can throw punches on the move and from various angles will have a definite advantage over a flat-footed opponent. This exercise is a basic drill designed to help the athlete get comfortable coming in and out of range. I find it best to start with simple punch combinations such as a Jab or 1-2 (Jab/Cross). As the athlete is completing their combination, just as they begin to bring their rear hand back to their chin, the coach will throw a light jab with the mitt towards the athlete's nose. The athlete will push off their lead foot while stepping with their rear foot to momentarily retreat only to quickly return into range with more punches. When this exercise is done at full speed for a 3-minute round, it will require a robust anaerobic capacity to maintain the movement and explosive energy. This in-and-out fighting style is beneficial and when combined with speed and a punch output will win you many bouts. A prime example in the professional ranks would be, Manny Pacquiao, who used this technique as he went up in weight to face bigger and seemingly stronger opponents.
Valuable coaching notes: Make sure that athlete doesn't open their guard by dropping their lead hand while stepping back. Also, watch for the athlete's rear foot while returning into range.
One bad habit that is important to avoid: Many boxers will "leave their rear foot behind" or drag it along as they only actually step with their lead foot to get back into range. Lastly, as the athlete fatigues, they will often start to lean back (or rock back) by just sifting their weight to their rear leg.