1) Have a good base. Making sure the lower body is stable and controlled is just as important in the having overall mobility in the trunk. If you aren’t stable, you may slide, lose your spine angle, or stand rather than turn, so number 1 is have a good base.
2) Mobilize your rib cage. The rib cage is pretty mis-understood and often overlooked when thinking about shoulder turn. Your shoulders sit on your rib cage, therefore in order to turn your shoulders your rib cage and spine must have the mobility to create that turn.
Here is one of my favorite drills to help increase mobility through the trunk and rib cage.
As I mentioned earlier, a good shoulder turn requires 2 things, a good base and good rib cage mobility. Make sure you keep your hips and pelvis stacked through the movement, and BREATHE. Exhale through your movement and if you get stuck or feel labored in your breathing, stop, hold that position and BREATHE into to relax and increase your mobility.