I have an opposing view on shadow boxing. When there's no target to stop your momentum, you stop it yourself, which in itself is a bad habit. Everything evolves. Just because it was done in the dark ages, doesn't mean it's always right. Like ancient Chinese medicine. Practicing moves and drilling in combos is good to a certain degree, but not necessarily everything about it is good. Just my opinion.

The best thing you can do is watch film of yourself. Or adopt moves from experienced guys and emulate them very slowly and repetitiously on the bag, which will develop the (good) habit as well.

For example...

When I first started I was dropping my left when throwing my right (typical of a newb). To correct it I'd start with my left hand out touching the bag (as though I just threw the jab), and then would throw the right slowly and properly while brining my left back to my cheek at the same time. ie, half the motion. Then I did the full 1-2 over and over and over, eventually at full speed. Problem solved and I can't do it any other way. Automatic now. And it didn't require a mirror.