Unless you are genetically gifted with very little bodyfat on your mid-section and have a flat nonprotruding abdomen, you will need to do more than just train your abdominal muscles to get abdominal definition. Most of you have a covering of bodyfat, water, and/or maybe a loose skin covering that hides your abdominal muscles. If you just train your abdominal muscles you may lose a few inches from tightening up your abdominal wall muscles, but you will not be able to see the muscle’s definition because the muscles are covered.
A good diet along with regular calorie burning exercise will help you reduce your body fat covering. The water covering can be shed temporarily by using some of the tricks of models and bodybuilders. However, they are usually not the healthiest of techniques. If you have loose skin that is beyond the point of snapping back, you can only be helped by the best surgeons.
Let me stay on track and talk about training the abdominal muscles. Generally you should spend approximately 10-15 minutes at least 3-4 times per week working your abdominals. A little less will keep the musculature strong but not in the tight shape that you want them. You also can work them longer and more often, but I feel it is not necessary. In each workout you should be able to do anywhere from 200 to 1,000 repetitions in total of various different exercises and depending on the exercises.
Learn how to do the exercises correctly. You can learn how to do them correctly from books, video tapes, or optimally from a fitness professional. Doing any exercise incorrectly is a waste of time and may be dangerous. Some exercises can add weight with the use of machines, free weights, and/or resistance bands. I recommend a weight heavy enough so you can do at least 20-25 repetitions. Anything heavier than that may put stress on your back.
If at any time you feel pain in your back, stop the exercise. Try a different exercise since there are so many. Also, the heavier weights may thicken your abdominals. Thicker abdominals are more visible, but that may not be the look you want. Try working with no weights or lighter weights with higher repetitions first for a flatter and slimmer look. You can always thicken them up later if you want to.
Try to work the oblique muscles on your sides first in each workout. Then move on to exercises that focus on the lower abdominals. Lastly, finish up with the exercises that focus on the upper abdominals. Change your routine every so often. There are many different exercises for the abdominals to keep you and your abs motivated.