Smile. Don't walk around all the time with a frown or even a neutral expression on your face.

学会微笑。不要总是愁眉不展，让脸部表情舒展自然。

youdao

They looked into the cameras with a neutral facial expression.

VOA: special.2009.11.24

It's no surprise that the doll, with its oversize forehead, neutral facial expression and enormous, alien eyes, didn't appeal to children.

FORBES: The Eyes Have It

In his ruling, Judge Sanders swatted aside free-speech arguments, saying the Texas statute prohibiting the unauthorized practice of law is "content-neutral" because it doesn't target the expression of any particular opinion.

FORBES: Arrest that software!