How to Tighten your Abdominals In order to tighten your abdominal muscles, you should: stand or sit with a tall posture, and hold your stomach in as much as possible. The best exercise you can do during the day to tighten your abs is . . . When you exhale; pull your stomach in, pushing the air out. Pull your stomach in tightly and a little quickly, and then let it out naturally while you breathe air back in the lungs. Pulling the stomach in tightly acts to push the air out of your lungs as you build your abdominal muscles.