If you are looking for one of the most effective stomach and abdominal exercises, think about adding planking to your routine! A plank, in any of it's variations, is an amazing exercise because it works so many abdominal (and other) muscles at the same time.
The abdominals are more than just what you feel when you touch your stomach area. They are actually arranged in layers that lie or wrap around your entire core. The more superficial layers help with movement and rotation, while the deeper ones aid in support and stability. They work in tandem, and when any part of them are weak, the whole system is weakened.
That's why endless crunches don't do the job! Crunches, for the most part, target the superficial rectus abdominus, or "six pack" layer. That's great, but it's only one part of the bigger picture and doing 300 of them, or even 3000 of them, won't give you a flatter stomach.
A flat stomach comes from a loss of body fat and a strong foundation. And added bonus of a solid core is a decrease in low back pain which can easily be caused by weak abdominals.

1. Lie face down on a mat or soft surface, resting on your forearms.
2. Push off of the floor, resting on your toes and elbows.
3. Keep your back flat, staying in a straight like from your head to your heels.
4. Contract your abdominals (pull your navel to your spine) to prevent your rear end from sticking up.
5. Hold for 20 to 60 seconds, and repeat 3 to 5 times.
For a modified plank, simply rest on the knees but still contract the abdominals and keep the body in a straight line.
So there you have it! There are many, many variations of planking, so brouse the internet and find a few others once you have mastered this beginning move.