What I did to work on my power was to work on my form. I would place my fist on the heavybag in the position where I wanted to finish my punch. After that I punched in reverse, I pulled my fist back and cam back to where I would start to throw my punch, and then I would punch regularly focusing on form.

Once form is taken care of then your muscle memory improves and your speed improves.

To add power put your weight behind your punches by using your legs and core muscles and not just your upper body