One Good Thing, Many Uses
The best part about One Good Thing is that it's a very fundamental heuristic, and it has so many uses. "One" makes it simple, "Good" makes it positive and constructive, and "Thing" makes it very flexible. :) So here are some uses -- use the talk page to suggest others if you come up with them.
Just Feel Better
When I'm feeling truly cruddy, and I have to dig deep down inside for some good cheer or determination, I end up using One Good Thing as one of the most reliable tools in my toolbox. All you have to do is pick one easy, simple thing, even something absurdly simple in a really bad moment, and just get that one task done, and you have changed your emotional momentum. If you try and fail, then the problem becomes more complex, but still not insurmountable, but this is a good reason to aim all the lower the first time. :) If you fail, your two best choices are to either analyze the failure, and work at what you can do different for a second attempt, or you can simply pick the first reason that comes to mind, and pat yourself on the back for at least trying. "But wait!" you say, "Isn't that just a rationalization?". Yes, yes it is. It still fixes the problem you set out to fix though; feeling like crud. A rationalization may be a small price to pay at this point, especially since you failed at tying your shoes, ya idgit. :)
To Err is Fully Human, To Give Up is Merely Human
If you've ever screwed something up, especially somebody else's stuff, you know that sinking feeling that you may not ever get it back to right again. If you haven't ever screwed up, then you are making your first mistake now, reading this. :) Broken teacups do not commonly reassemble themselves, and entropy is a measure of their stubborn refusal to do so with any great probability. So break out the superglue and give it your best energetic and informed try anyway. Or go and buy a new teacup. Or something. One Good Thing will suffice to start the ball rolling here. Try it, you can't possibly fail, right? :)