The crunches are good exercise to strengthen your abdominal muscles:
The basic crunch. Lie on the back on an exercise or yoga mat with your hands behind the head, knees bent, feet flat and slightly apart on the floor. Using the abdominal muscles (by contracting them), raise the shoulders off the floor. While going up, round the back and roll the spine upwards. Slowly come down to the initial position.
Reverse crunch. Lie on the back with the arms extended downwards towards the feet and palms on the floor. Place the hands under the lower back & hip. Legs bent at 90 degrees up in the air. Using the abdominal muscles .i.e. contracting them, roll the spine & the hips upwards so that the knees come towards the face. Slowly go down without changing the angle of the legs.
Pilates. Pilates is a form of mind-body exercise, aimed at strengthening your core muscles, using your body weight. One can perform unlimited body positions in Pilates such as seated, supine, standing, kneeling etc. It is functionally useful and very effective for maintaining proper form and alignment for various activities of daily living and sports needs.
Important principles of Pilates are proper posture/form, quality of movement over quantity, proper breathing, effectively establishing mind-body connection and slow-to-gradual progression.
Benefits of Pilates include increase in functional strength, improved flexibility, increase in core control and core strength, improved posture and better movement quality and helps in keeping spine strong, stable and injury free.