Captain's Chair.

At the gym: Using a captain's chair, stabilize your upper body by gripping the handholds and lightly pressing your lower back against the back pad. Start by holding your body up and letting your legs dangle below. Tilt your pelvis forward, and slowly bring your knees toward your chest. Lower your legs only until they form a 90-degree angle with the floor, before lifting them again. Keep the motion of raising and lowering your legs very controlled.
At-home version: Sit up straight in a firm, armless chair. Grab the chair's edges just in front of your hips. While supporting yourself with your hands, slowly draw your knees up toward your chest, keeping your lower back pressed against the chair. Hold, then slowly lower. (To make this move more challenging, start by using your arms to hold yourself suspended in the air.)