For flat toned abdominal muscles I found a great simple work out. These moves work out the tops, bottoms the middle and sides of your core so will result in a nice flat stomach for the summer.
As you get stronger you will be able to do more in less time or you may do more for a longer period of time. To increase the intensity you may want to slow your movements down. You may want to experiment and perform the exercises faster.
Here is the crazy ultimate core workout, as illustrated by Women’s Health magazine: