Go read these tips from The Happiness Project. Seven really simple things. Here's my favorite:
3. Rid yourself of a nagging task: answer a difficult email, purchase something you need, or call to make that dentist’s appointment. Crossing an irksome chore off your to-do list will give you a big rush of energy and cheer, and you’ll be surprised that you procrastinated for so long.
I do this one already but never thought of it as something to do to boost my mood. Now I know! 🙂