After doing any stomach exercises it is very important to perform a lower ab stretch. Not only will this help to prevent DOMS (delayed onset of muscle soreness) but it will also help to ensure that you don't develop bulk in the abdominal region and that you keep it flat and firm.
One of the best ways to do a lower ab stretch is to simply start by laying on your stomach. Then, slowly press your body upwards using your arm muscles until you feel a slight pull in the bottom portion of your stomach. Note that you shouldn't push yourself too far with this as you are also contracting your spine and pressing that too far to the extreme is never a smart idea.
Take a few deep breaths in and out while performing this lower ab stretch and then slowly come back down to the floor once again. Repeat the entire process two to three more times.
After finishing, flip over onto your back and pull your legs towards your chest. This will help to curve your back in the opposite direction in case any stress was placed on it while you were bending backward.
Another exercise to perform is a simple side bend. Don't neglect this exercise thinking that it will only target the side muscles. If you bend sideways and slightly back you will also target the bottom stomach as well. Just be sure you are holding onto something while performing it so you do not lose your balance.
So don't forget to include either one of these exercises after you perform your stomach strengthening moves. It is a wonderful way to cool down after a hard workout.