It's often said that smiling takes more muscles than frowning-- an interesting possibility, but not a great reason for smiling because it probably takes even fewer muscles to hold a completely neutral facial expression. A more encouraging interpretation of this saying is that smiling is easier than frowning because we tend to do it more often.
When I go outside on a cold morning to walk to a lecture or to the grocery store, I sometimes find myself moving my face as little as possible just to see how neutral an expression I can keep. This is best paired with my generic t-shirt, jeans, and jacket, and a 10-yard stare, the municipal alternative to the 1000-yard stare. It's my idea of what might blend in with the crowd.
After a minute or two in neutral, I kick things up a notch-- 1 second smiling, then 4 seconds rest, and so on. I sometimes smile at people, but I hear that in some cultures eye contact with strangers is supposed to be like staring into the sun, so I generally just smile at pigeons instead. I don't know if this blends in with the crowd any more or less-- when you're looking at pigeons, you can't see what other people are doing.