I'm not an expert, but still listen to me.
To add power to a punch you can do one or all of a few things:
1.) Increase the mass (of your hand, arm, body, gain weight, etc.)
2.) You can increase the velocity of the movement overall, or by increasing acceleration of the movement
3.) You can increase the efficiency of the movement (technique) so that you don't lose the ability to maximize your mass/maximum velocity/acceleration and therefore the force you generate.
I think that sums it up for the most part. For me, it's about working the technique slowly and rerouting the neuromuscular pathway to be as "correct" as possible and then augmenting it with weight training enhancers to increase the amount of force you can generate.
Also, for me, endurance is a big deal. I can throw a pretty solid left hook when I'm fresh, but if my endurance is lacking my left hook sucks after a few minutes of heavy workload. I'd rather be able to hit 70% power 90% of the time than 90% power 50% of the time, if that makes sense.
I have a couple of these Bulgarian bags and have been using them more, really focusing on exploding through the motion.
I also do a lot of sideways medicine ball slams.
holy shit that looks exhausting