I would guess you’re asking about a comment you might make when you’re saying goodbye to someone. I don’t think I’ve ever heard anyone say, “Keep a smile,” and while there’s nothing specifically wrong with the sentence, it sounds incomplete to me. “Keep a smile on your face” is possible. We do hear, “Keep that smile,” if the other person is already smiling, and “Keep your chin up” is sometimes said.
“Good point,” he said, wiping his mouth clean with a napkin. “All right, just check to make sure it’s all right with your folks and I’ll let my mom know she can take down the ad in the paper.”
“In the paper?” I laughed. “Don’t you guys have access to the internet?”
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