There are some very specific things you have to do inside the ring to be a good boxer, but there are also some very specific things you should NEVER do in the ring in order to be a good boxer. Of course, there are the basic rules, like...don't drop your jab when you bring it back, don't step with the wrong foot first, be sure to keep your hands up and so on. Aside from some of these types of common, basic mistakes, there are a few more advanced ones that should be taken very seriously. Consider them the Cardinal Sins of Boxing.
Sin #1. Never, ever drop your hands when you are coming out of an exchange. If you step back with your hands down, you are almost always guaranteed to get clipped. Instances where this has come back to bite a fighter are endless, but for a perfect example, type Mike McCallum versus Donald Curry into YouTube and, at least, watch the fifth round to see the last, biggest mistake a once-promising fighter ever made.
Sin #2. Never step straight back when you go on the defensive. Step side to side, give your opponent angles and force them to adjust their attack to find you. When you move straight back, you are staying right in the line of fire and right on the end of your opponents punches...the last place you want to be.
Sin #3. When you have just ducked and slipped a combination and are coming up from a crouched position, come up throwing. Don't just expect your opponent to stop throwing and let you stand up to engage again. Transform it into an offensive move by immediately retaliating from your defensive position and turning the tide back in your favor.
Sin #4. Never reach out to block punches. Make your opponent come to you. Make him commit to the punch, commit his weight and then counter. When you meet the punch half way, you make your opponent's job too easy. Again, if you want a perfect example, look at the fourth round of Lennox Lewis versus Hasim Rahman on YouTube. The third round says it all.
Sin #5. Don't lead from the outside with an inside punch. The uppercut is an inside punch. The hook is an inside punch. Floyd Mayweather has a slick and effective left hook lead, but only because he uses it correctly. He uses it sparingly, he uses it selectively, but he is also Floyd Mayweather and there's only one of those. As a general rule…don't do it. Don't lead with a hook or with an uppercut. They are outside punches and take too long to reach their mark. Instead, properly set them up with lead punches and sprinkle them into your combinations to make them most effective.
Once you're inside the ring, there are a myriad of mistakes you can make. Any one of them may be minor and have little or no effect on the outcome of the fight. Then, there is the other kind. It is major. It can be dramatic and it can work against you, sometimes spelling your defeat. No mistakes are good, but breaking any one of the Cardinal Sins of Boxing and you're opening yourself up to making a major faux pas. It is usually unforgiving with bad consequences and you won't get away with it very often. Confess to yourself or to your coach that you will never be tempted to do any of these. There is no good in them. Now go and sin no more.