Guy's Wandering Eye in Public?

HELP! My boyfriend can't sometimes help looking at other pretty women when we are out. He doesn't do it obvious, it's like some subconscious thing. I know that guys can't help to look, but what would be a tactful way of getting back his attention without making too much of a fuss? There are times I wanted to smack him or other times I cleared my throat loudly but it seems I am hen-pecking him....