From the Howstuffworks.com website:

Why diets tend not to work

The reason why most diets tend not to work for very long is because they are not sustainable. A person gains weight because he or she consumes more calories per day than needed. The diet creates a temporary deficit. When the diet ends, the person goes back to normal eating and the weight comes back.

Let’s look at an example. Say that you weigh 150 pounds. That means that you burn 1,800 calories per day in a resting state. Let’s also imagine that in the course of a day you burn 200 more calories living your life — walking up and down steps, carrying in the groceries and so on. Your calorie needs then are, on average, 2,000 calories per day. Now let’s further imagine that, on average, you consume 2,050 calories per day. On a daily basis your body is taking in, and therefore storing, 50 calories more than it needs. So every 70 days (3,500 calories in a pound / 50 calories each day = 70 days) you gain 1 pound (0.45 kg). If that “50 extra calories per day” trend continues, then over the course of a year you would gain 5 pounds. This, by the way, is the pattern for a big portion of the U.S. population. If you over-consume by just a few calories per day, over time you will gain weight. Keep in mind that just one Oreo-type cookie contains 50 calories, so over-consuming is incredibly easy.

Building a sustainable diet

The first step to building a sustainable diet is to start counting the calories that you consume in a day so that you become conscious of two things:

* You need to understand exactly how many calories you are eating on a “normal” day.

* You need to realize where each calorie comes from — you need to build a calorie database in your brain so that you know, whenever you eat something, just how many calories it is supplying.

In the United States, any food that you buy in the grocery store is required by the U.S. Food & Drug Administration to have a nutritional label with that food’s calorie content. You can also look at a chart like this one to find out the number of calories in different foods. Any chain restaurant will supply you with nutrition information both at the store and on the Web.

The second step is to figure out how many calories you need in a day. You can use the “12 calories per pound” rule, or you can get more precise by looking at the formulas in How Calories Work.

Pick your “ideal weight” — the weight that you would like to maintain. Then calculate how many calories a day you can consume to maintain that weight.

The third step is to compare the two numbers — You may be startled by the difference between the “number of calories you need” and “the number of calories that you take in” in a day. That is where the extra pounds are coming from.

The fourth step is to figure out how to bring the two numbers in line. What you will soon realize is that 1,600 or 1,800 or 2,000 calories per day just isn’t that many. You have to watch and count everything you eat and drink every day and stick to your daily limit.

The fifth step might be to add exercise to the mix so that you can raise the number of calories you can consume per day. Online resources like this exercise calculator will show you how many calories different forms of exercise can burn. Burning 250 or 500 calories per day through exercise can make a big difference.

Read more:

Cutting Calories - making wise food selections

Fitting in exercise

Weight loss myths
And even more stuff…here.