This little "mood hack" has been the subject of ongoing research, and it has supporters and detractors just like many other theories. The idea is this: if you're feeling a little down, putting a smile on your face will trick your mind into thinking that you're happier. As a result, you'll feel happier, and eventually that smile will be more genuine.
Research on this idea has been conducted in several ways. One of the most notable studies had people clasp a pencil in their teeth, forcing their mouth into a smile. Try it! Just as shallow breathing sends a panic message (something must be wrong! I'm breathing rapidly!), a smile sends a message of improved mood. The most damning research in opposition is pretty weak stuff, mostly saying "we don't think it works all that great."
So, let's try this: either put a half-smile on your face and keep it there for a minute, or better yet, hold a pencil in your mouth, using your molars to clamp down on it. Your mouth will be pulled back into a smile, your brow will change, and you'll FEEL the smile on your face. Hold it for a minute or two, and see what happens. At worst, you'll just have the taste of pencil in your mouth for a while.
At best, though, you'll have a learned a ridiculously easy way to get a little mood boost.
An interesting offshoot of this facial feedback research is the mounting evidence that botox treatments can actually impact mood and cognition due to LACK of feedback. But that's another story for another day.
For now... smile!