i don't think it's what pieces you are playing. i think it is practicing slow, medium, and fast (starting out slowly with the weight of your arms just as when they are at your sides when you walk). your teacher may have you play lighter the faster piece you play, but this doesn't negate the neck and shoulder relaxation. your arms really don't change that much either. i think it is your fingertip touch that you are aiming for. you want to hear the notes clearly, right?! if you play too soft, you cannot distinguish the notes as much. just let your hands feel as though they are massaging the keys when you practice slowly, and then as you speed up you make more minimal movements at your fingertips, but still purposeful. i hope this is right?! i haven't played some of the more difficult pieces that you are.