I first started wrestling in 1997, my freshman year in high school, at 145 pounds. That was 11 years ago. Minus my hiatus while recovering from shoulder surgery, I have since competed continuously in sports governed by weight classes. I wrestled as heavy as 184 pounds (while only weighing in at 174) and as light as 143 pounds (my measurements at the time were 40-28.5-39). These days I lift at 77kg (169.7 pounds) and usually weigh 79kg-81kg with a waist size of just under 32 inches (31" on weigh-in morning). I have probably "made weight" well over 100 times and never once failed to weigh at or below the magic number.
I think this qualifies me to tell you a thing or two about how to control your weight.
While a 50-year-old book I have says that it's a myth that exercise helps you lose weight, it's true that participating in athletic activities will help keep your weight down. People, especially my mother, love to tell me that I'm slim because I'm so physically active. Usually they tell me this when I'm on my third lunch of the day. But since my sophomore year of high school I have weighed everywhere between 141 pounds and 205 pounds, purposefully working to get to either extreme. There were times when I'd run or wrestle a little extra to lose weight, but even when I gained weight I didn't actually train any less - I just ate differently.
Don't get me wrong - you really, really, really, really, really should exercise. Both aerobic and anaerobic exercise are necessary to keep you in shape. Weight training is generally anaerobic, but it also has aerobic benefits, Olympic weightlifting especially. It doesn't matter if you are male or female, it is equally beneficial. If you are one of those people who says this sport is bad for your joints overall or says that it is bad for women, I am officially calling you out for being stuck in the 1950s. You may as well have written that diet book I mentioned. You're like the old golf and baseball coaches who thought lifting was bad until Tiger Woods came along and proved weight training makes you better at, well, everything! Also, unless you follow a program specifically designed to do so, weightlifting won't make you gain a significant amount of weight, ESPECIALLY if you're female. Women just don't physiologically tend to build much muscle. Female bodybuilders are taking male hormones. Period.
You can't easily change your body's base metabolic rate, so food will always remain the major factor in your weight. While it's true that the difference between calories in and calories out divided by 3500 is the number of pounds you gain, this deceptively simple equation has made fools out of just about everybody. Everyone sees how they can control calories in, but nobody knows how to affect calories out. So, people read nutrition labels to count calories and fat (because fat equals BAD, right?). They strive to eat less and less, and not only do they tend to fail in the process because it's tempting to cheat or not count calories fairly, but they tend to fail because they're unwittingly lowering calories out! If you normally eat 2,500 calories a day and burn 2,400 (a net gain of 1/35 of a pound) and instead eat 2,000 calories a day and burn 1,800, you're actually gaining weight twice as fast! And don't forget that if you go back to eating what you used to, your body will also burn less than it used to for awhile, and again you'll gain weight faster than before you started. If all you think about is cutting calories, you ALWAYS lose.
But you CAN change calories out. In fact, it's not all that hard! First, you need to be aware that your metabolic rate increases when you eat and then later decreases. If you pay attention, you can actually feel it changing - when it is higher than normal, you feel more energetic, and when it is lower you feel more tired. You tend to feel worse afterwards if you eat too much or eat too crappily. What you want to do is keep that rate as high as possible as much as possible and eliminate the lows. Not only will this help you burn more calories, you'll spend more of your day feeling happy and you'll have more energy. The trick to keeping it high is to eat often enough to get that increase and avoid the decrease by not eating too much at one time. Of course, what is "too much" depends on what you're eating - two McDonalds double cheeseburgers will probably make you feel pretty down, though you could eat the same weight in bananas and feel like a million bucks.
A simple example: in my opinion, a good burrito from Chipotle should have rice, black beans, meat, tomato salsa, corn salsa, sour cream, cheese, and lettuce. That's easily over 1,200 calories. Let's say you eat two per day and only drink water otherwise. If you eat them in one meal, you'll get your 2,500 calories, and you'll spike your metabolic rate and then fall asleep on the couch in a stupor. Since you won't want to even move for awhile or do anything strenuous for the rest of the day, chances are good you just put on weight. You could eat them in two separate meals, instead. Each time you have one you'll be tired afterwards (unless you have a rather large stomach), but your day isn't ruined (unless you have a rather small stomach). You may or may not gain weight at the end of the day, just because you were able to get up and move around afterwards. Now what if you cut them in half and ate one piece every four or five hours? After each meal you'd probably feel peppier than before, you'd get more done, you'd move around more, and by the time the day is over you will probably have lost weight.
That's right: the same food, eaten differently, can be used to gain weight or lose weight.
People seem to think that eating five or six small meals a day is the province of bodybuilders and powerlifters, but it's by far the best way to stay healthy, slim, and, even more importantly, feeling good and productive all day. I KNOW it works, because I monitor my weight closely and watch it go up when I enjoy a few too many big dinners in a row, and I watch it go down even when I'm not eating the healthiest as long as the meals are small. The week before a contest, I usually eat MORE. I just eat lots of complex carbohydrates, cut out sugar (and salt, because, sadly, water weighs), and eat almost all day long. I'm not saying you should do that, because that mode isn't really satisfying, but there's an old axiom in all intensive athletics which says that if you're thirsty, you're already dehydrated. I think the same goes with hunger - if you're REALLY hungry, you should have eaten earlier.
But don't take my word for it. After all, it's not like I have any experience in the matter :-P.