Many ways. I like leg lifts but even better, grab an exercise ball. Using an exercise ball for stomach toning gives you stronger muscles than if you did the exercises on the floor. Plus, you get bonus benefits, like better balance. Start with the Ab curl: Sit on the exercise ball, feet flat on the floor, knees hip-width apart and bent at a 90-degree angle. Place your hands behind your head, elbows pointed out, and slowly roll back until your mid to lower back rests on the ball. Pull your ribs toward your pelvis to raise your upper back 3 to 4 inches off the ball. Hold for a second and then return your shoulders to the ball. Repeat 10 times.
With your exercise ball you can also build back muscles with the supine bridge: Sit on the ball with your knees hip-width apart and bent at 90 degrees, feet flat on the floor. Roll, so that your shoulder blades are resting on the ball and your torso is suspended above -- and parallel to -- the floor. Like a bridge. Tilt your pelvis, so that your bottom dips toward. Leg lifts are also great. For this all you need is a flat surface, like the floor: Lie on the floor, on your right side, with your right arm extended in front of you for balance and support. Keep your torso straight, aligning your shoulders and hips so that they form a straight line with your ears. You can bend your right knee slightly for balance. Keep your left leg straight.
Once you’ve mastered these exercises, add resistance. Using weights or resistance bands increases the intensity, which burns more calories. And don't forget exercises like lunges and squats, which strengthen your core by placing them on a firmer foundation. Finally, believe it or not, moving your arms behind you might be better at making your trunk muscles contract than stomach exercises. So give it a try: Stand with your back to a wall, arms at your sides. Turn one palm against the wall and push it into the wall (keep your arm straight). Hold for a few seconds, release, and repeat.