Aside from the typical sit up, there are a number of abdominal strengthening exercises that can help strengthen your abs and, in turn, reduce lower back pain.
Start by lying on your back on a supportive yet firm surface, such as a yoga mat or carpet. Draw your legs up so your knees are planted flat on the floor. Cross your arms over your chest so that you’re lightly grabbing the opposite shoulder.
Take a deep breath in and tense the muscles in your abdomen. Slowly raise your upper body, keeping your back straight. It’s important to use your abdominal muscles instead of simply curling your neck. Be sure your upper body is far enough off the floor that your shoulders are not touching the ground. Hold the pose for several seconds, then slowly lower back to the floor. Relax and breathe out.
Repeat the exercise multiple times. As you gain strength, hold the pose for longer and do more repetitions.