Lately I have started to appreciate 10 m circles before doing shoulder-in. I must feel that Colonel is vertical and bent around my inside leg. Inside leg must feel like a pillar, very vertical from the hip.I can direct his shoulders around the circle with my both hands. I control his hind end with sitting well down on my outside thigh and my whole outside leg behind the girth. If I have that, usually, shoulder-in starts very up in front, very cadenced, no loss of rhythm. It is easier for me to do right shoulder-in than left because my left seatbone is down more than my right. It is easier to keep your light seatbone down when it is on inside of the circle than when it is on outside. Outside thigh and seatbone are very important because if you neglect them, you will push your horse's ribcage to the outside so much his body looses verticality and he will drop his inside shoulder. This way you shape your horse between your thighs. Circle will help you to establish that before starting shoulder-in. Happy riding...