Tactically agree, strategically disagree.

I'm not going to say anything about genetics, metabolic rate, energy input/output ratio and all. They're already well known by everyone.

What I'm going to point out as the most effective thing (genetics aside) you can do for lifelong slim body is that, you need to shrink your stomach. Filling your stomach with "light food" such as vegetables or no-nutrition artifical foods (we call them "hardboard" here) is one of the worst methods to lose weight. This method keeps your stomach large, actually enlarges it even more. And the moment you leave your tormentous diet, you're again tete-a-tete with your ever large stomach. As long as you have a large stomach, you will need to eat waay more than what you actually need. This is a closed circuit. The more you eat "hardboard" (much eating with less energy intake) the larger your stomach will get, the more hungry you'll feel, the more you'll need to eat. Break that chain!

That means you need to leave the table half-full while slimming; and "nearly full" from then on. Treat the pleasure of feeling %100 full the way you'd treat feeling "loaded" with some intoxicant (nicotine, alcohol, etc). Just like smoking or alcohol, you need to "quit" getting full.

With a small stomach, you will feel full with just a fraction of what a fat person eats. A small stomach will not let you eat much. It's impossible. You will explode. But, and this is a very important BUT, if you start leaving the table getting full (counting on the fact that you eat drastically less anyway) then you stomach will start to enlarge 0.01% each day, and then you will find that you need to eat more and more to feel %100 filled. Don't do that. Eat whatever/whenever you like, but never to the point of feeling full. Do it life time. Adopt this as a lifestyle. After a while you'll get accustoned to feeling half-full all the time, and gradually forget the pleasure of feeling 100% full. It's like quitting alcohol or smoking. Back it with some moderate but regular sports, and then you're done: lifetime slim and healthy body.

So, instead of going after no-nutrition foods (aka stomach enlargers), I would've gone after nutritionally rich ones, so that I would leave the table hungry but still get sufficient nutrition and energy.

The keyword is "shrunk stomach".