It's not magic...

...it's common sense.
As a species, our nutritional needs evolved hundreds of thousands of years ago. Therefore, we have certain innate signals that guide us toward a diet that is healthy:
Thirst--drinking water is good for you!
Cravings--often specifically responsive to a developing deficiency;
Feeling "stuffed"--if you feel it you probably have eaten too much, in calories as well as volume;
Hunger--research shows that people who ONLY eat when they are hungry tend not to be overweight and people who eat based on external cues --time of day, emotions, peer pressure--often are;
Your body will usually not lie to you, but others things may. Trust your "gut"--and maybe you won't get one!