Don't sink your stomach ..

[hitesh70]

My friend usually pull his stomach inside to avoid looking fat.

How do I tell him not to do that?

- Don't take your stomach inside.
- Don't sink your stomach.

Which one is more practice way to say this? i use second one..

[emsr2d2]

Don't suck your stomach in.
Don't pull your stomach in.

Neither of your suggestions is used.