First, I will write a new blog entry today that will contain more detail on this topic, so if you are serious about losing weight, I bet it will be worth your 10 minutes to visit my blog later today and read the post.
Here's the quick and dirty:
You gain weight for exactly and only one simple reason: you intake more calories than you expend over a given time (such as a day). The surplus of energy is stored as fat at a rate of 3500 calories per pound of fat. So if you burn 2000 calories each day, and you eat 2500 calories each day, then every 7 days you will add one pound of fat. The converse is also true. If you burn 2000 calories each day, and you only eat 1500 calories each day, then you will lose one pound of fat every week. It really is [almost] that simple.
Your body is going to be reluctant to burn fat, and prefer to slow down the rate of caloric output (your energy use) before dipping into the fat reserves. So when you eat less, you will feel like you have less energy. So in reality, if you burn 2000 calories each day, and then suddenly you start to eat only 1500 calories each day, your body is going to first respond to this new imbalance by turning down the throttle. You will feel less energetic and you will probably reduce your energy output each day unless you do something to actively maintain or elevate your energy output. This is called exercise.

Your body and mind work together to trick you into eating at least as many calories worth of food each day as the amount you expend. So if your metabolic rate is 2000 calories per day, I guarantee you that you will intake at least 2000 calories per day unless you do something to intentionally measure and control your caloric intake. So you can't just decide you are going to only eat fresh fruits and vegetables plus steamed fish and expect to lose weight. You can't just say "no more cheesecake" and expect to lose weight. You mind will figure out a way to replace the cheesecake calories with fruit calories and you will continue without any change except maybe your general health might improve. It is possible to be very fit and also fat.
Simply put, you are not going to lose weight unless you intake fewer calories than you expend. If you intake fewer calories than you expend, then you will feel less energetic and you are going to have to do something to force your metabolic rate to stay high enough that your body has no other choice than to burn fat. The easiest way to do this is to begin the day with an hour or more of aerobic exercise. Get a heart rate monitor and calculate your estimated max (220 minus your age). Spend an hour or more at average 70% of your max heart rate and this should do the trick. If you do this in the morning before you eat anything then you will already be operating in your fat metabolism (since it has been many hours since you ate). This will kind of "jump start" your metabolism.
While you are doing this, you need a method to measure your metabolic rate, and then you need to deliberately control your calorie intake. Your brain is going to interpret different stimuli as "hungry". For example, I think most fat people think they are hungry if they are not "full". And most fat people feel like they are "starving" if they can sense ketones in their blood (ketones are a by-product of burning fat). You have to re-train yourself to interpret "full" as "stuffed", "not full" needs to feel normal, and you need to be able to tolerate extended periods of ketosis if you want to lose weight. In other words, you have to ignore your feeling of hunger, and eat based on a plan and not your impulses.
There are ways to measure your metabolic rate but until you get within about 20% of your ideal weight, or unless you are a trained athlete, it doesn't really matter. You can estimate. Assuming you have a relatively sedentary lifestyle (you are not an athlete, you work in an office), then you can use the following formula:
Men: 1.2 x [66 + (6.2 x wt in lb) + (12.7 x ht in inches) - (6.8 x age in years)]
Women: 1.2 x [655 + (4.4 x wt in lb) + (4.6 x ht in inches) - (4.7 x age in years)]
If you are more active than just "desk job", then you can probably increase the 1.2x up to maybe 1.4 unless you are a fitness nut, in which case you don't need to be reading this.
So let's say we have what I'd guess is our average TX CHL holder: 5'9" male, 230 lb, 35 years old.
1.2 x [66 + (6.2 x 230) + (12.7 x 69) - (6.8 x 35)] = 2556 calories/day metabolic rate.
Now make a plan for how quickly you want to lose weight, with a max of about 1% of your body weight per week. For this person that would be 2.3 lb per week. Since each lb of fat contains 3500 calories, then for each pound you want to lose per week, you need to reduce your caloric intake by 500 calories each day. So you calculate your daily calorie budget by subtracting the amount you need to cut from your metabolic rate:
daily calorie budget = metabolic rate - [(lb/week goal) x 500]
daily calorie budget = 2556 - (2.3 x 500) = 1406 calories/day
So now start reading those labels and counting calories. Figure out how you are going to get by on 1406 calories per day! That's probably half the food you are used to eating! But if you do this, you will definitely and absolutely lose the weight.
Here are some tips that lead to success:
1. don't eat anything with an unknown calorie content. Measure the food you cook and prepare with scales and measuring cups. Skip things that you can't nail down the nutritional information.
2. come up with a regular menu of stuff you like to eat with known calorie values. Always eat from that menu so you know your calorie content.
3. don't drink anything with calories. This includes milk, beer, sodas, gatorade...
4. skip desserts. they are not worth it. you'll eventually lose your appetite for them anyway.
5. identify high calorie-density foods and skip them. eat low calorie-density foods instead. like, eat a carrot instead of a tortilla. a carrot is more filling and has 1/4 the calories. a whole head of lettuce is only about 50 calories.
6. decide what you are going to eat, particularly if you are eating out, before you are hungry. then stick to that decision. so if you are going to pick up lunch at Wendy's, then go to their web site in the morning, read the nutrition info on their menu, and pick what you are going to order for lunch based on the calorie content and mix of protein, fat and carbohydrates. then when you go to lunch, don't read the menu, just order what you already decided to order.
7. increasing your exercise can dramatically increase your metabolic rate, which will allow you to eat more and still lose weight. Just as you can be fit and still be fat, you can very easily be thin and not at all fit. Losing weight won't make you fit. It takes daily exercise.
8. Log (write down) every single thing you eat, and record an accurate calorie count. An Excel spreadsheet will work, but so will a simple note pad you can carry in your pocket. This is a very critical, important step!
9. whatever you do, do it every single day.
Now after you lose the weight you wanted to lose, if you just quit counting calories and get back to an impulse-based diet, you will just gain the weight back again, at the same rate that you gained it in the first place. Odds are you are going to have to count calories and deliberately reduce your calorie intake for the rest of your life if you want to stay thin.