Okay so what I'd suggest as a quick exercise to help your form is to stand in your boxing stance, now rotate your upperbody to the opposite stance. Keep your legs in place - now aside from looking ridiculous, what you have is your shoulder all the way back and your rear hip coming forward. You should feel a shift in your weight and balance. Now from here just extend your arm to hit the bag a few times. Once you feel like you're balanced in that goofy stance arm punching, go back your normal stance and throw a punch. You should feel your shoulder come all the way back and your hips to shift. You'll notice a big difference immediately.