If I meet someone's eyes when out in public, then I smile. I smile at cashiers, I smile at people talking to me, I even smile at dogs, goddammit.
I rarely smile at children...strange. But yeah, I smile quite a bit, but I walk around without a smile. I'm usually listening to music when out and about. Usually Lacuna Coil. It takes all of my concentration to not spontaneously break out in song and start throwing myself around shopping centres and whatnot like a loon.
I'm a happy person. I think more people should smile. ESPECIALLY cashiers. God damn, they take your money and are STILL miserable.