Do not try to lose weight too rapidly. Crash diets and diet pills that promise weight loss are usually bad for you and actually don't help keep the weight off in the long run. Resist the urge to take the "easy" way out and instead stick with a healthier lifestyle. This way you lose the weight and improve your health, helping you keep the weight off in a way that won't harm you in the long run.
For even more impressive effects on body composition: aim for exercise forms which elicit a positive hormonal response. This means lifting really heavy things (strength training), or interval training. Such exercise increases levels of the sex hormone testosterone (primarily in men) as well as growth hormone. Not only do greater levels of these hormones increase your muscle mass, but they also decrease your visceral fat (belly fat) in the long term.
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.