There's basically two schools of thought:
Theory 1: Punchers Are Born, Not Made
Pretty self-explanatory. Ppl like Mike Tyon, Cus Damato, Teddy Atlas, Kevin Rooney, etc, etc, all subscribe to this theory.
Theory 2: Punchers Are Made, Not Born
Jack Dempsey, one of the all-time-great Heavyweight boxing champion subscribed to this theory. Basically, Dempsey said that, through better technique, timing, speed, and skill, one could amplify his power and thus acheive a harder punch.
Good technique comprises of utilizing your quads (thighs), hips, glutes, back muscles, and pectorals in addition to proper form to deliver a devastating strike.
Futhermore, speed, timing and counterpunching skill can also intensify the power of your punch. As boxing champion Sugar Ray Robinson once said, "the hardest punch is the one you don't see coming" (or something to that effect). Basically, if you slip your opponent's punch, or throw your punch at the right time, you can catch him off guard and deliver an knockout punch.
Also, throwing your punch at fast speed can increase its impact (momentum = mass x velocity). The faster you throw your punch, the more momentum it will impact with.
From personal experience, I can tell you that working on technique/form is the best chance a fighter has of improving his punching power. Once you have the technique/form down, work on improving your speed and you should notice the difference in your punches.
At the same time, keep in mind that the "Punchers are born" theory is also a valid one. Many professional boxers have gone their entire careers without developing a knockout punch (keep in mind that these are people who box for a living. Sometimes a person is just not capable of knocking another man out).