What are some good exercises to flatten the stomach?

Sit ups every day. I have been doing them for five years now and have a gorgeous flat stomach. Also work out at least twice a week and ride your bicycle a lot. Eat only three types of carbs a day. Eat healthy, drink lots of water and love yourself :)

One thing I've noticed, having a flat stomach, is that running gives me a stomach. I read to find out why, and it said stuff. I don't remember what. But I can tell you I've noticed this.

Put the fork down

get a nice rope and skip lunch

Sit-ups help.

Plank