Psychology researchers discuss their findings in a New York Times op-ed piece.
The take-home advice:
Positive thinking fools our minds into perceiving that we’ve already attained our goal, slackening our readiness to pursue it.
...
What does work better is a hybrid approach that combines positive thinking with “realism.” Here’s how it works. Think of a wish. For a few minutes, imagine the wish coming true, letting your mind wander and drift where it will. Then shift gears. Spend a few more minutes imagining the obstacles that stand in the way of realizing your wish.
This simple process, which my colleagues and I call “mental contrasting,” has produced powerful results in laboratory experiments. When participants have performed mental contrasting with reasonable, potentially attainable wishes, they have come away more energized and achieved better results compared with participants who either positively fantasized or dwelt on the obstacles.
When participants have performed mental contrasting with wishes that are not reasonable or attainable, they have disengaged more from these wishes. Mental contrasting spurs us on when it makes sense to pursue a wish, and lets us abandon wishes more readily when it doesn’t, so that we can go after other, more reasonable ambitions.
I like that article. For people capable of thinking about what methods make humans happy, it seems unlikely that simply performing any feel-good method will overcome barriers as difficult as what happiness means or what use is happiness anyway. They might improve one's outlook in the short term, or provide an easier platform to help answer those questions, but to me the notion that therapy works because of therapists (a sort of research supported idea if I recall correctly) corresponds well to the intuition that humans are just too wrapped up for overly easy feel-good solutions to work. (This is as opposed to psychiatric solutions to psychiatric issues, for which you should be following this algorithm if you're depressed).
I've had trouble with the notion that happiness is even a goal to be strived for at all, because of the self-referential reality that a really good way to become happy is to become less self-focused, but that thinking about being happy is sort of self-focused. In that sense, I'd much rather seek out "fulfillment" or "goodness" than "happiness," but I now think that my issue here is just an artifact of the language of people using the word "happy." That word is just too wrapped up in ideas that make it out to be something like wireheading, which as we know is something that nobody actually wants. And so while I do think people looking for X often stop short with not-very-desirable things, it's good to separate this from people who actually want to be the most good kind of happy, the kind that one would always want, and maybe still even call "happy."