Try this exercise.

Sit up straight in a chair. Keep your shoulders and hips square while you alternately put more weight on your right seatbone and then your left one. You’ll feel each seatbone press down (heavier, deeper) into the chair. Once you can alternately press each seatbone down into the cushion of the chair without leaning left or right, try that same feeling on your horse.

Click on Program Your Position for more tips on correcting your riding position.