My initial response is on the front page, but it's older and not very deep, so I figured I'd add some stuff that I've learned since that has really helped me to develop my explosiveness and my standup skills.
Do explosiveness training. It's been said that machine based or free-weights based power training is the best thing to develop punching power, and that's the school of thought that older guys like Tank Abbott come from, but I think, personally, that that's a load of crap. Jumping pushups and explosive burpees will help your cardio and increase your strength training. On top of that, they will give you the pop that you need standing up to put guys to sleep.
Focus on stiffening up your straight punches. The jab and the cross are the two most common punches, and if you make them sting then you're pretty solid in the standup department. Even if you don't put guys to sleep, making sure that these punches land stiff can put you in a good position to follow up on opponents and get TKO's.
Work on your follow throughs for hooks and uppercuts. There is a whole ideology between the followthrough that it has to be short, but powerful. That's definitely something requisite for a knock out. While your physical power will play into it, what will also play in is how you follow through on the punch. The better your followthrough, the more damage the initial impact will do, and it's one of the examples why TKD and Karate guys don't transition so well MMA, because they work more for speed and snap than for that followthrough on their hooks and uppercuts.
Really balance your arsenal. If you've got a good knowledge of a large group of punches, you can set up the ones that you know will put your opponent to sleep with ones that you know will, at the very least, do some damage. It also helps you learn to control angles and attack from different positions that some standup guys don't know how to do.
Hope that was helpful for somebody.
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