This helps for me.
Lie on your back, both knees in the air at a 90 degree angle, extend you legs out straight infront of you, so they are parallel to the floor but a few cm off the floor, repeat this over and over. The slower you work the better for the stomach muscles. If you find it hard doing both legs at the same time, do them alternatly.
You also need to make sure you don't hurt your spine (if you do Pilates, you need a nuteral spine) if not, put a few fingers under the arch of your back when lying on the floor, and make sure you dont lift too far up, or drop down too far. If any of this hurts your back then don't do it.