There are many ways to work on your abs. The goal of dynamic movements (crunches) is to sculpt your abs, while static movements (strengthening) work on the deep muscles.
The right position
Remember to contract your abs and support your neck, lower back, and core while doing your exercises. Don't overemphasise the range of your movements: it's better to keep your range of movement small but controlled. Maintain a smaller angle to the floor and tighten your abs! If you have back trouble, take it slow and easy.