How do you prevent your body from storing fat?

I think there are some "common sense" answers but no magic bullet:

Avoid processed foods e.g. refined flours and sugars - eat more natural foods such as fruit, vegetables, meat occasionally. Refined products flood the body with excess calories which are stored as fat.

Eat little and often rather than all-you-can-eat blowouts. The body will store any excess calories as fat. Smaller meals help to prevent this.

Do some regular excercise to raise the metabolic rate i.e. fat-burning. It also gets the blood pumping and will much improve all-round health.

As I said, there's no magic bullet that would allow someone to eat anything they liked and not gain weight - and if there were it would probably damage your health in much worse ways. I know it sounds boring but basically the middle-road works the best.