One weird thing to do that may actually work: Refuse to be optimistic (if you're just not a positive person). In an experiment run by the two authors, Heidi Grant Halvorson, PhD, and E. Tory Higgins, PhD, "play-not-to-lose" people (who also tend to be pessimists) were given a word game. Halfway through the game, they were told that they were succeeding—and promptly lost their motivation and stopped performing. On the other hand, when they were told they weren't doing so well at the same halfway point, they got inspired to do better, solved more puzzles and had a more enjoyable time. All of which leads us back to the old happy-ism—be yourself. Even the positively negative you.
Next: The rules of happiness you know (but keep forgetting!)