Any good core muscle program should incorporate abdominal exercises that are performed from a variety of different angles. Such exercises will be sure to target all of your abdominal fibres to be sure you are getting the best workout.
A few good exercises to perform in your core muscle program that hit the rectus abdominis are front crunches on an exercise ball, front leg raises, V-sits (bringing both arms and feet up together so your body forms a "V") and then hanging leg raises. This, coupled with a proper diet that gets you losing bodyfat, will help you develop the striations on the front of the abdominal section.
Next up in your core muscle program you should be doing exercises for your obliques. These are the muscles that are located on the side of your stomach and are responsible for the twisting actions you perform. A great exercise for the obliques is started by standing back to back with a partner. With one of you holding a medicine ball in front of your stomach, twist to the side so you can pass the ball off to your partner (they are twisting around too in order to receive the ball). Once they have it in their position, they will now twist to the other side of their body and pass it back to you. Repeat this process 10 times or so and then repeat in the other direction.
Lastly, you will want to perform an exercise for your lower back. Supermans are an excellent choice as you can do them anywhere without any equipment. To start, lay with your stomach facing the ground. Then, with arms extended out in front of you, lift the legs and arms up as high as you can and hold this position for five to ten seconds. Lower back down so you are almost touching the ground and then perform a second rep. Try to aim to do ten or twenty at a time.
Like any other body part, be sure to rest at least a day in between training session to allow for rest and recuperation.