• Crunches: Lay down a mat and lie down on it. Bend you knees as much as you can. Touch both your elbows to each other while keeping your palms lightly on your ears. Keeping both your hands behind your head can cause stress to your lower back and wont be good for you in the long run. Now using only your ab muscles raise your upper half of the body toward your knees. Try to touch your elbows to your knees. Do not lift your entire back from the floor, as this causes back strain. The flexing of your ab muscles when lifting your shoulders off the mat is the most important part of the exercise. Just as you lift off, pause for a moment and exhale all the air in your lungs. Inhale slowly as you go back to your original position. Breathing right is vital to building muscles. As you repeat don’t let your hear rest on the floor.
• Sit Ups: Performing sit ups regularly is another great way of developing those ab muscles. Lie down on your mat and place your fingers behind your ears. Rest your feet on the floor and bend your knees. Lift yourself completely, from the shoulder blades to your lower back. Lower yourself once again. Keep repeating. This will eventually become easy, when it does, add challenges. Strap some weight to your chest or use an inclined bench.
• Leg Lifts: While performing sit ups, lift your legs up into the air. Alternatively move your legs like you would while pedaling. If you keep your hands behind the head, then bring your knees up to touch opposite elbows, that is, right elbow to left knee. Another kind of leg lift is to lift your leg 90 degrees up in the air without bending your knees. Now lower your legs (without bending you knees of course) and repeat.
• V-ups: Lay down on your mat and straighten your legs. Place your arms above your head. Now raise your torso and legs at the same time without bending you kneed. Try to reach out and touch your toes. This will be tough but will be possible with enough practice. Hold the position for a few seconds then relax and repeat the whole exercise. If needed add some weight to your feet.
• Jackknife Sit-ups:  Once again, lay down on your mat. Keep your hands on your sides to maintain balance. Now raise your torso and knees at the same time. During this exercise you legs will fold automatically and touch your feet to your hips, looking like a jackknife. Relax your position and repeat. If you think you can handle some more pressure add more weight.

Once you start performing the above exercises regularly you will start noticing your abs develop. Now you might want to train you oblique muscles too. Oblique muscles are the ones on the sides of you stomach. Any sort of twisting against some resistance will help develop these muscles. You can use twisting machines found in gyms or twist while performing sit ups. Alternative exercises include side bends and using a medicine ball to twist your sides. Remember, your oblique muscles wont be as strong as your abs and hence you should take it easy when starting off.

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