A lot of people aren't kind to their low back or neck when they do crunches.
To do the move right, think about pulling your ribs down towards your pelvis while you're performing a crunch. Keep your neck and shoulders relaxed, and squeeze your shoulder blades together. Your chin can move slightly towards your chest, bur you don't want to jerk the neck around. Maintain good contact between your low back and the floor while you're crunching, and don't rush it.
Reverse crunches are also a great way to hit all your ab muscles all at once. To do these, lay on the floor with your hands out to the side. Lift your legs into the air, bending them so that your calves are parallel with the floor and your knees are over your hips. Then exhale, contract your abs, and raise your hips off the mat, as if you were trying to bring your knees toward your head. Keep the 90-degree angle of your knees steady. Hold the move briefly at the top, then slowly lower your knees back to their starting point above your hips, with control.