The internal and external obliques, which I like to call "nature's girdle," are located at the sides of the rectus abdominis. They are the muscles you use when you bend sideways at the spine or twist at the waist. They also contract to compress the abdomen, so you should work them just as hard as you work the rectus abdominis. Adding a twist to crunches works, as well as dumbbell side bends, but be careful not to use any jerky motions, especially if you've had back problems.
Sure, you can lose weight quickly. There are plenty of fad diets that work to shed pounds rapidly -- while leaving you feeling hungry and deprived. But what good is losing weight only to regain it? To keep pounds off permanently, it's best to lose weight slowly. And many experts say you can do that without going on a "diet." Instead, the key is making simple tweaks to your lifestyle.