Diets don't work. This is very well established. In fact, if you're analyzing whether any given person is likely to gain weight in the future, the most important indicator is if they have previously been on a diet. Those who have been on diets are more likely to gain weight in the future than those who have not.
If diets are not successful, why are they constantly recommended on Oprah, in magazines, in bestselling books; in fact, seemingly everywhere? For the simple reason that telling people what they want to believe is often more profitable than the truth. People want a shortcut; any kind of short cut that avoids real thought and prolonged effort. They would rather take a pill than be careful about their health; they would rather have surgery than exercise; they would rather gamble than make sensible long term investments. Whatever Americans want, they seek the fast food version: shallow, fast, and easy. But the human body does not really care what people want. It has its own reality. When you look at any diet, the only question is whether you can continue it indefinitely; in other words, for the foreseeable future. Better yet, for the rest of your life.
In terms of body weight, all that matters is a very simple equation. If you take in more calories than you burn, you will gain weight. If you burn more calories than you take in, you will lose weight. (To understand what a calorie really is, and how they are measured, click here.) If you engage in a fad diet that dramatically lowers your calorie intake, you will lose weight during that period, but as soon as you go off the diet your body will return to the equilibrium dictated by your current caloric intake and expenditure.
Limit your portion sizes, forgo junk food entirely, do some exercise every day, or as many days as possible, and you will weigh what you should weigh. It's really as simple - and as hard - as that. The secret to success is not any particular diet or exercise, but daily, long-term discipline. That is boring, and it won't make anyone rich. It's also true.