Developing a lean, flat stomach takes time and patience, especially when it comes to those last few kilograms in those hard to lose places. There is only one way to lose fat in the so-called 'stubborn areas,' and that is with the correct combination strength training (with a extra focus on mid-section), cardiovascular exercise (short, hard workouts), and stable blood sugar (keeps you from adding additional fat and makes it easier for the body to use body fat for fuel).
Cardio, cardio, cardio - Doing all the abs exercises in the world will do nothing if you have a layer of fat covering it. Doing strength training, or lifting weights, would help, but not as much as aerobic exercise. If you want to add some ab exercises in after the cardio, that's great, but be sure to work your whole torso, not just the upper abs — that includes the lower abs, lower back and the muscles that wrap around your sides.
For example, Walking, jogging, bicycling, elliptical exercise and stair climbing are all great fat burners. Most give up early in the workout. During the first 10 minutes of aerobic activity, glycogen (stored carbohydrates) is the primary fuel source. Because you don't burn significant amounts of fat until glycogen stores are depleted, the key to maximum fat loss is to work out aerobically for 30-60 minutes continuously per session.
Diet – Its best to stay away from fried food and too many sugary desserts. Abdominal fat loss is a 50% exercise, 50% nutrition combination. Regardless of how much you workout, if the number of calories you take in is greater than the amount you burn, you'll still put on abdominal fat.
Eat Small Meals Regularly - You should try and spread your calories out into five small meals a day instead of 2 or 3 big ones so that you don't overeat in one feeding.
Give it Time – If you want to have a flat stomach in 3 weeks, forget it. Losing fat takes time, and it's unhealthy to lose too much weight too fast. Gradual weight loss is healthier, and more likely to be sustained over time. Go for a lifestyle change, something you can live with for the rest of your life, or you will just yo-yo. It's a marathon, not a sprint.