perma-smile
The insincerely constant smiling expression on the face of a politician or other performer, which is meant to ingratiate oneself with voters/fans, as opposed to being a true expression of the person's feelings.
Headline of a political commentary article: "Mitt Romney: the man behind the perma-smile."
"Because she's always wearing that perma-smile, it's hard to know what's really going on with her.
"Because she's always wearing that perma-smile, it's hard to know what's really going on with her.