You can do 1000 crunches every night, but if you aren't paying proper attention to your nutritional habits and getting regular aerobic exercise, you will likely never achieve the proverbial "six-pack." Remember that the abdominal muscles rest beneath a layer of fat, so the best thing you can do to lean up your mid-section is make sure you aren't overdoing your daily intake of calories. The other side of the equation involves expending calories through moderate to vigorous exercise most days of the week.
You should also keep in mind that the stomach is a very common place for the body to store fat, so even if you are successful in reducing your total amount of body fat, you may never see those rippled abdominal muscles, simply because your body seeks to keep a certain amount of fat there. That doesn't mean, however, that crunches are useless. On the contrary, keeping your core muscles toned has many benefits, including improved posture, increased overall strength, and perhaps decreasing your risk of back injury. When doing abdominal exercises, you don't have to just stick to the standard crunches on the floor; there are many variations you can and should try.
A good place to start is by getting a big fitball (it looks like an overgrown beach ball) at your local sporting goods store; they usually come with an instruction sheet showing a variety of different core workouts to do. Here are three good exercises you can do with a fitball:
1. Standard crunch: Sit on the fitball, then walk your feet forward so that your low back is on the fitball but your shoulder blades are not. Place your fingertips behind your head to support your neck. Keep your gaze toward the ceiling as you lift your chest up toward the sky, using your abdominal muscles. Slowly return to the starting position. If the positioning bothers your back, make slight adjustments in the placement of your back on the ball until you are comfortable.
2. Oblique twist: Lay on the fitball as in the above exercise. This time, as you lift your chest up, twist slightly so that you are aiming your shoulder for your opposite knee. Return to your starting position, and on the next crunch, twist to the other side.
3. Reverse crunch: Lay on the ground with the fitball between your lower legs (so you are holding the ball by squeezing it with your calves). Lift your legs up in the air so that your hips are at a 90 degree angle. If that bothers your back, bend your knees as much as you need to alleviate strain. With your arms at your sides to help you stay balanced, lift your hips up off the ground slightly (so your legs and the ball go up in the air and your pelvis comes off the ground). This shouldn't be a big movement; the greatest contraction of your abdominals will occur with the initial lift, so you don't need to swing your legs way up in the air (as this can be counterproductive).
Also, remember that doing a zillion repetitions of an exercise doesn't necessarily mean your getting more benefit. In fact, you are better off doing a smaller number of good quality repetitions where your focus is on your form. I would recommend aiming for two to three -of 15-25 repetitions each. Then, as you get stronger, rather than adding on to the number of repetitions, find other abdominal exercises that are more challenging. There's no shortage of ab exercises out there! Mixing up the exercises you do will keep your body stimulated and ensure that you are working all the muscles in your torso to their fullest.