I have experimented with this and smiled a half smile when walking among people. So often, I receive smiles back. This doesn't have to be a big, teeth baring grin. Just a gentle smile, like Bibbity's smile above.
And, surprise, if I am in a bad mood and smile the half smile just for myself, my mood seems to lighten. This isn't meant to be a grin and bear it kinda thing. There are times when crying, or being miserable seem the right thing to do to express my feelings. But, what is the harm in giving a half smile a whirl? Try this: experiment with the half smile and see what happens. Share your findings here if you wish.
above sketch of Bibbity is one I drew during last years "Big Draw."