I've always kept a book of things that make me happy. I have this little journal and in it I write with colorful pens (as colors make me happy) little things that amuse me or things I can do to pull me out of a blue mood. Such as I really enjoy rain, and the quadratic equation, Gone with the Wind, and an 8 oz coffee cup really amuses me.
Other than that, a bath is always helpful. I envision my troubles and doubts going down the drain as I let the water out.
sometimes when I'm feeling down it's because I'm bored, so a walk always helps. I read somewhere that if you directly forward and even slightly up when you walk, it elevates your mood. I find that when I do this I notice more, like the trees against the sky down town, the shape of the buildings and the colors of all that is slightly more up than the ground. That always makes me smile because it reminds me of the beauty I'm surrounded by.
coffee always boosts my mood too. Drugs will do that though.
Having more plants around me, like in my bedroom or throughout the house makes me feel better as well.
The simple act of smiling, even if you don't feel like it, sends messages to your brain, rather your psyche and will subconsciously make you feel better. The power of suggestion is powerful and one can suggest to the self just as much as any one else.