Many aspects about taking a punch are just as mental as much as physical. You have to be able to re-focus pain, or ignore pain for a split second or two. Let's face it, getting punched is going to hurt, but you can lessen the degree of pain and damage to your internal organs by doing the following things:
1. Think "gravity" instead of "body builder." The last thing you want to do when getting punched is to stand up as tall as you can and puff out your chest like you are in the military. When you stand tall it spreads out your ribs, exposing your internal organs more, but even worse, it creates a very thin muscle barrier between those organs and the punch. You want to push down and out. Pull you shoulders down toward the earth. Tighten up your abdominal muscles as much as you can, but also push your stomach out. This creates a much larger space between the fist and your internal organs.
2. Get killer abs- Not only is core strength important to daily health, but it is also critical to taking a hard punch. You need to develop the ability to tighten your core in a split second and to react as soon as someone throws the punch. You can practice tightening you abs for long periods of time as you walk around or sit at work. Try to concentrate on your breathing while you are doing this. Do not hold your breath. This will take some time to master.
3. Work on your breathing- in order to react quickly to taking a punch, you must have control over your breath as well. As soon as the punch makes contact, let out a small, strong breath of air in the time it takes to say "ha." You will tighten up every muscle in your core region as hard as you possibly can for just that split second and then move back to a relaxed state.
4. You must be able to punch back- If someone is going to hit you that hard you should not just have to stand there and take it, unless you are trying to win a bet or something. You will need to defend yourself with a punch that will make the attacker remember never do that again. The quickest and best way to develop a fist of steel is to use makiwara training. A makiwara will train your fist to make rock-solid contact without injury, and most importantly, trains your bare knuckles. You most likely will not be wearing gloves when you need to punch someone back.