I recently went from long, waist length hair to shoulder length. Do I need to change the amount of shampoo I use?

When shampooing, we only need to use enough to rub through the scalp, because as you rinse, the shampoo will pick up any dirt through to the ends. If you have been shampooing only your scalp, then you do not need to change the amount of shampoo that you currently use. If, however, you shampoo the rest of your hair as well as the scalp, then you are definitely using too much and need change the amount of shampoo that you use to focus only on the hair through your scalp. Also, if your hair won't behave after washing, it might simply be that you are using too much shampoo and stripping all the oil from the hair. For shoulder length hair, pour a 10 pence size amount of shampoo into the palm of your hand. That will be plenty. For shorter hair, simply adjust the amount. Don't expect a mountain of lather. It really isn't necessary anyway. And imagine the savings on your shampoo bill!