Yeah good point from Silent Assassin, trying too hard will make your punches slow and weak. So long as you have strong technique and you work on your strength and speed you cant really do alot more, if there was a specific thing that would give you that one punch power everyone would be on it, but there isnt, in much the same way that some people can take a punch better than others, there are bits and pieces you can do to strengthen your neck and get better flexibility, but in most cases, the ability to take a good punch, as well as give one is something that people have before they ever step foot inside a boxing gym