Writing down a letter of gratitude can substantially improve happiness for months
This is only one out of many studies showing this. Writing elaborate (at least 300 words) gratitude letters is perhaps the easiest and nicest thing you can do to improve happiness. Extra points of happiness for someone else if you actually go there and read it out loud to them :)
That first article gives the results of a pilot study for the second linked paper so probably shouldn't be considered additional evidence.
EDIT: Also, the results of the second study show that for the post writing period both the letter-writers and the control had the same response (within a half unit of standard deviation).
Why don't we have Life Hack thread like we do have Media Threads and Rationality Quote Threads?
Well, now we do.
I'm copying in the comments some hacks previously written about by others for your convenience, also check out the Munchkin post if you haven't, there are many interesting hacks there. Timewise, I've made it quarterly because it is harder to face a life hack than it is to face o quote, so less frequency may improve average quality. Below is a suggested description for them with structure taken from media threads. Any further ideas on how to improve the thread are welcome and should be made at the Metathread tree.
This is the quarterly thread for posting Life Hacks of various types that you've found that you enjoy. Four main thread-trees will be in the comment section.
1) There is substantial evidence for it (please post pointers to it)
2) I tried/ Friends tried/ There is some small evidence for it
3) Seems like a cool idea
4) Metathread
Rules: