Crunches are a great way to increase abdominal muscle strength, but to strengthen core muscles you need to address all the inner and out abdominal, and back muscles. Two that will help you to do that are:

Get into a four point position, on your hands and knees; knees directly under the hips, hands directly under the shoulders. The back should be in a neutral position, chin slightly tucked. Draw your belly button into your backbone without rounding your back, extend your left leg (keeping your hips focused toward the floor) while extening your opposite (right) arm....almost as though someone was pulling on you from each end, hold for a pause, then exhale, and with the next inhale, extend the other leg and opposite arm. Do about 6 - 10 repetitions, slowly and with purpose.

Roll over onto your back. Pull your knees directly above your hips into a 'chair position". Your arms should be extended on the floor next to your hips. Draw your belly botton into your backbone and then do a slight pelvic tilt, which will press your lower back into the floor (keep your shoulders relaxed). Hold that for a pause, relax, and then repeat slowly and while breathing deeply for up to ten repetitions. On the last repetition, hold the backbone pressed into the floor for a count of eight then stretch your legs to one wall the arms to the other.