How to get a flat stomach?

Now, I don’t want exactly want to lose weight. I just want a flatter stomach. I have sort of have bad posture that causes me to have a little tiny bit of a gut. But I am skinny, and I’m happy with my weight. I just want a flatter stomach, one the i don’t have to suck in all the time.

And & Y E are there tips on how to walk with good posture.

Thanks (: