If you can't turn your lower body independently of your upper body (the pelvic-rotation test), then you'll likely swing "over the top" of the target line from outside to inside on a steep angle. This causes sliced or pulled shots. You want to swing down from inside the target line.
Keeping the upper body facing forward, hold a medicine ball and step up, rotating your hips to the side.
Hold onto a club and stand on one leg. Keep your shoulders square and rotate your pelvis back and forth.
Lie on your side, legs together. First separate your legs at the knees. After a set, separate your legs at the hips and repeat.