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.
Obviously, it’s still possible to lose weight on any diet – just eat fewer calories than you burn, right? The problem with this simplistic advice is that it ignores the elephant in the room: Hunger. Most people don’t like to “just eat less”, i.e. being hungry forever. That’s dieting for masochists. Sooner or later, a normal person will give up and eat, hence the prevalence of “yo-yo dieting”.
Here's something else most people probably don't know: Fidgeting is good for you. It's considered a nonexercise physical activity, and it's an important way to burn energy. You get more health benefits if, in addition to exercising, you are a more fidgety, more active person the rest of the day. This means gesturing while you're talking, tapping your foot, just moving around.