“First and foremost you have to train your abs on all three planes. You have to train them forward and back with something like a crunch or double crunch. You have to train your abs laterally to hit the obliques with something like a dumbbell side bend. And you have to train your abs on a rotational plane—your transverse abs—and you can do that from a seated trunk twist."
Want a flatter stomach in two seconds? Stand up straight! Slouching emphasizes belly rolls but straightening your spine elongates your whole body, making you look taller and sleeker. Want to go even flatter for a picture? Use the old modeling trick and arch your back slightly—this will pull your skin tighter across your stomach while moving it farther away from the camera, making it look slightly smaller. Yeah, it's a temporary fix but good posture offers many health benefits beyond looking good.