Stand with your back against a wall. Put your feet shoulder-width apart, and away from the wall. Slowly slide your back down the wall. It may take time, but work towards being able to get to the point where your thighs are parallel with the floor -- like you're sitting in a chair. Hold for 5 to 10 seconds. Hold for longer as you get stronger.
Repeat 3-5 times. Do 3 to 5 times a week.
Exercising muscles that AS affects -- in your back, shoulders, neck, butt, and hips -- can help you move more easily.