pay attention to what you're doing
See watch your back.
While walking down the street you go to turn down a blind alley. At the corner someone notices you.
"Hey smarten up!."
You look down the alley realizing what you were about to do and proceed down the street instead.
to smarten somebody up - to make look nicer or neat in appearance
then he smartened her up
he would smarten up her (ignore this line, it's just for the stupid input mask to stop saying "Example doesn't use the word smarten up."
purportedly, kidnapped child Natascha Kampusch's kidnapper would try to smarten her up and photograph her when he had beaten her up. (Don't like this example though it's the source - from Wikipedia. Better example anyone ?)