If you also have a problem with rounded, or hunched shoulders, forgo crunches altogether, as they tend to increase the curvature of the upper spine. Instead, from either the supine position (lying with legs straight), or from the relaxed, hip flexor-supported position (with knees bent), press your low back into the floor by contracting your abdominal muscles, hold then release. Keep your entire lower body relaxed. Your arms should be held out in a tee position, palms up. Perform two or three sets of 10 to 20 repetitions with a brief hold (or you can do one set of two or three repetitions with a 10 to 30 second hold).
Wall Standing is a variation on the pelvic tilt. Stand with your back flat against a wall, heels out at least six inches. Keeping your shoulders and pelvis against the wall, press the low back into the wall with a strong abdominal contraction. The closer to the wall you are with your feet, the more abdominal effort it will take to flatten your back. Hold for 10 seconds up to 1 minute.
The above combination of exercises, if done properly, will flatten, tone, and tighten your abdominal muscles, improve posture and appearance, and possibly relieve symptoms of low back pain. Of course, no amount of abdominal work will remove the layers of fat you've accumulated over the last 20 years through overeating and under exercising. A properly orchestrated strength and cardiovascular program, combined with sensible eating is the best way to achieve that.