You could try this, wont give you bad Knees. guy doing it at the time, had never Boxed. http://www.youtube.com/watch?v=WScy2GqIEyE
lol....me too....although, i think i'll stick with tried and true methods to increase punching output....and by that of course, i'm talking about ROCK JAW!!!
Punch more during workouts. Your body is accustomed to working at a certain output; increase the output and make your body adapt to that.
increase your shoulder endurance. Just throw non-stop 1-2s on the bag. Do it for a timed amount. Do this for 3 rounds each session. Each week, increase the time. When you stop progressing with time, drop the time back where you started, and add another round. Then increase time again each week as before. Rinse and repeat.