Smile!
I believe the notion that simply smiling will make you feel happier. Sometimes when I'm feeling a little down, I smile. Just smile. Even if it is more of a grimace. Imagine somebody saying something funny and smile in response to it. Or think of a really funny part in a movie or show you like. Who cares if you are in the middle of a crowd, or at work, and people might think you are a little strange if you start smiling or laughing for no obvious reason. I'd rather be thought crazy than be in a bad mood. Personally.
One thing that seriously makes me happy is, when driving by myself, turning up the music and just singing to it. Oldies songs are the best, but any song you like. It is best when you are alone because there is no reason to be self-conscious about your singing. When I am singing strongly, my voice cracks and I am terribly off key. But do I care? No. I blast it and at the end I just laugh so hard that I don't care if school was stressful or work was terrible.
Or if you are in a less serious situation (so not in class or at work or such), talk in an accent. I'm terrible at them (I usually start in an English accent, slide into a Southern, and occasionally contribute some Russian).
Here's some of the best medicine: try to make somebody else smile. Because smiles and laughs are contagious, and if you make others smile, you'll get one pretty quickly.
