Not that abs are my strong point, but first you need to burn excess body fat with a proper cardio routine to define the area. Three to four sessions a week of intense jogging, running, the Stairmaster, the elliptical, jumping rope, or biking should be enough to get the process in gear. Swimming, hiking, and taking aerobics classes are beneficial as well. Nothing beats jogging or running. It is the most intense, efficient, and effective method of burning calories.
Next, you need to pay close attention to nutrition. Nutrition is important because you can reduce the amount of calories you take in, therefore greatly having an effect on the amount you have to expend through cardio. Eat five to six small, well-balanced meals spaced apart about every three to four hours. Try to keep something healthy on hand. If not, when you become real hungry, you will opt for something that isn't as good for you. When you become overly hungry, all rational thinking goes out the window. Also, this may sound a little excessive, but drink at least a gallon of clean water each day as well. It will help in nutrient absorption and digestion and will help flush toxins from the body. Bottom line, make sure you are supplying your body with well-balanced, healthy food every three to four hours.
For exercises, I suggest doing weighted crunches, cable rope crunches, weighted leg raises, using the seated ab machine and stability ball crunches.
This, all in combination, should give you a strong, well defined midsection. Good luck.