My tip - do three sets of 15 instead of one.

If the crunches hurt your back and neck then you are probably pulling. Do not lace your fingers behind your head, actually if your neck is hurting try not putting them behind your head at all, then you wont be tempted to pull which means less likelihood of neck pain. As for your back, try pulling your hips toward the sky, i was always told to do that, its something about the curve in your spine. Try it out, hope it helps.