If you want flatter abs, work on your plank.

The plank doesn’t really work the “six-pack” muscles. It works a deeper muscle called the Tranverse Abdominis. That muscle works like a weight belt. It helps in keeping your spine stiff while you move a weight.

Work it post workout.

So just like a weight belt, the Tranverse Abdominis holds your gut in. And the plank will help you build endurance. This makes it easier to maintain your abs tight during the day. Because just a few sets a week will not cut it. You need to suck it in during the day so that it becomes more natural.