A fitness ball is a useful item for core training. Most gyms have fitness balls available, or you can buy one to use at home. There are entire books dedicated to fitness ball workouts, but these three moves are some of the most effective.
Stability ball twist: Lie with your back on a fitness ball and your feet flat on the floor. Hold a small medicine ball between your hands and extend your arms up toward the ceiling. Keeping your butt on the ball, slowly rotate your torso to the right so that your arms are parallel to the floor and your knuckles are pointing towards the side wall. Hold to exhale, then slowly rotate back to the center and repeat on the other side. Start with a few repetitions to each side and work up to 3 sets of 10 repetitions.
Back extension on the ball: Start by kneeling behind the ball with your hands behind your head. Lean forward and lift your knees off the floor, curling your toes under so you are balanced on your toes with your torso resting on the ball. Raise your chest slightly, and then lower it down while keeping your spine straight from the tailbone all the way to your neck.
Crunch on the ball: Start by leaning back against the ball so it is supporting your torso, while your knees are bent and your feet are flat on the floor. Place your hands behind your head and curl upward so your shoulder blades lift off the ball, then lower your shoulders down.