I agree that using a mirror is the best solution for this. Got this guidance from a pro a couple years ago and it really transformed my swing. Specifically, he said to put a piece of tape on the mirror aligned to your nose (so vertical). Take your golf club and lay it across your upper arms, and then fold your arms so that your hands are resting on the opposite shoulders. This holds the club in place. Now assume your stance facing the mirror. Practice rotating back and through concentrating on keeping your nose aligned to that tape (both back and through, though eventually on the through you will come up and out). You'll see right away when your head moves back. Note your head can and should turn a little, but your head should not move back. It also should be noted that unless you're a very low handicapper that you should attempt to keep your head vertically level as well throughout. Only the best athletes can do the dip move with consistency, so he suggested I go for maintaining consistent head heigh as well. It will feel very strange at first, but repeat that 20 times for a few days in a row and then take it to the range. Good luck.