One good exercise that can strengthen both the abdominal muscles and the back is the pelvic tilt. Lie on your back on the floor. Bend your knees bent so that your feet are parallel on the floor (in line with your hips). Keep your arms at your sides. You will notice at this point that the small of your back is off the floor. Tense your abdominal muscles so that the small of your back is pressing into the floor (your lower torso will be slightly raised). Hold this position for 5 seconds, then relax. Start by doing this five times per day, gradually working your way up to 10 repetitions per day.
This is another exercise that will work your lower abdominal muscle and help stabilize your back. For a reverse crunch, lie on your back with your hands underneath your bottom, and rest your legs straight out on the floor. Keep your back flush to the floor and tighten your abdominal muscles. Keeping your legs straight, slowly raise them up into the air so that they are at a 90-degree angle with the floor (and your body), making sure that your back stays on the floor. Slowly lower them back down until they are just about to touch the floor, and then raise them back up. Do five reps per day.
It is also important to keep your upper abdominal muscles strong to prevent back pain. A simple exercise for this is the trunk curl. Lie on your back with your arms crossed against your chest. Place your feet on the floor, keeping your knees bent. Then, slowly lift your trunk about 15 degrees off the floor. Your goal should be to raise your chest up in the arm while keeping your neck straight. Hold this position for 5 seconds, and then slowly lower yourself back down. Start by doing this five times a day, gradually working your way up to 10 repetitions each day.