Also, when you do crunches make sure you sink your stomach into the floor, and you only need to get your shoulders off the ground like an inch, otherwise you can hurt your back. If you're doing it right you will feel the burn. But keep going it's worth it!
I think the most important thing about crunches is to get that reverse flexion by using an exercise ball. You will get more contraction in the muscle having to come up from leaning back and you don't have to go up as far.
Leg lifts, incline sit ups or leg lifts are great for lower abdominals