The simplest way I can suggest as to how to punch harder is to:
First get your form right(angle, trajectory) in the most comfortable way for you.
Heavy bag is always good to get your wrists used to the impact and impact stress.
Knuckle pushups are a forgotten method to strenghthen the Knuckles.
My personal method is to aim for the back of the head when punching in the face, this induces a natural follow through that greatly increases the power of your punches, use this for other targets as well.