Get Happy.


Here's the thing. Pick up a pen. Place it between your teeth (horizontally, we don't want a Nosferatu look going on) and then let go. Bite the pen gently, and continue biting down. Feel the shape your face is making? Look in the mirror. It is using the same muscles that smiling uses. Do it for long enough, your muscles send signals to your brain. They tell it that you are smiling. Brain reacts accordingly, and cheers the hell up. 
It works.