When you're feeling depressed for no good reason, force yourself to laugh. It triggers happiness almost as well as externally-induced laughter. Eventually, you will noticeably condition yourself to release seratonin (or endorphins, or something) every time you notice that you're seratonin-deficient.

It's been effective for me. I started it as a moody teenager and it quickly became self-perpetuating. Google suggests I'm not alone. It's got a whiff of wire-heading, I admit, but ideally you're using it to solve a brain chemistry defect, not an external problem.