Obviously hitting the heavy bag. I used to do loads of powerlifting, had a pretty good total in competition too. It doesn't do anything much for punching power though.
Some guys do weight exercises at my gym. I do none, and hit harder than all of them. It's definitely not weights. Maybe that can help you optimize your technique to put a little more weight into your punch? To me, punching power is all about form, technique, and the shoulder snap.
I've been looking for a good book on Joe Louis training/techniques. What was the one where you read about his techniques?
I saw one of Jacobs a few days ago, consisted of him throwing medicinal balls at walls in punching motion i will try it.
It's called Joe Louis: How to Box That book differentiated me from everyone else, in such a good way.
Someone mentioned technique is more important, i totally agree. But i would go over the technique with small weights.
Do power clean and snatch if you are to use weights. Deadlifts Push ups Burpees and of course bag work
As Abel Sanchez described it, like when you snap a towel, here is a more detailed description: http://www.expertboxing.com/boxing-techniques/punch-techniques/how-to-throw-a-snapping-punch
When you snap your punches they make a nice loud crisp sound, the power is not so much going through the target as landing on the target and quickly coming back - the sound gives it away
power comes from your core and the shoulder. Legs aren't a necessity to hitting hard.the twisting motion from mid body is what generates the power .For shoulders just do alot HB work and punch high level and do in between periods with a speed bag hung high where you are reaching up to burn the shoulder , if possible. Dont overwork shoulders they will grow with bag work anyway. Do lat excercises because its a huge muscle that helps drive the punch and reinforces it. Forearm and wrist ,this will help absorb impact and make you hit harder( I get my power from my Forearms...G. Foreman), you can only hit as hard as you hand can support which is supported by the wrist/forearm. bicep curls /resistance training bicep routines will help hook punches. All in all when throwing a punch everything has to work together and that's shoulders/lats/bicep and thats alighned with the core twisting of the hook and proper leg stance that will get you the most power,and you always punch through the target when throwing maximum power because in real life you are trying to break bones or knockout someone. look at video footage of who you think is a power hitter you resemble and overall you tube vids from beginners hitting to coaches to fighters,you learn more visually then reading a book, unless you have the power concept down already. I know we had our differences Optimus Prime but gave you solid advice(REAL ) advice anyway.