I was on the bus to work this morning, and a comment my wife had made once suddenly clicked. She said sometime during the weekend that she's starting to get wrinkles (which I don't believe for a moment, but anyway) and that they're the kind of wrinkles you get on the corner of your mouth when you've been smiling too much.
During my commute, I noticed how people really don't smile. Not only that, but their mouths have been arched downwards for so long that it is their natural state, which makes them look even sadder. The act of smiling on its own can make you feel better, and it definitely makes you look better.
So, even if at a specific moment you don't feel like it, remember to smile every once in a while.