Use! This! One! Simple! Trick!
Say you’re trying to decide where to put your wallet or your car keys, or what directory to save a file in. Ask yourself this: “When I’m looking for this in the future, where will I, logically speaking, look for it?” Put the thing there. For example, I have a spot for my wallet, which I take whenever I leave my house. Since I often also take my cars keys when I leave the house, next to my wallet is a logical place for the keys. (And cell phone, etc.)
This is an obvious example, but it can be used on less obvious things, things you don’t think about as much. I’ve successfully used this to quickly find things I don’t look for often, e.g., in my office at work. Say it’s an old document I only need to look at every couple of years. There are only a few logical places I might have put that, and I usually can find it quickly by second-guessing my past self.
Furthermore, it gets even easier once you get into this habit, because in the future you’ll think, “Where would my past self, thinking game-theoretically about making things easy for me, have put this?”
This is particularly helpful for things you don’t retrieve a lot. For things you use every day, the old standby “A place for everything and everything in its place” is unbeatable. But you won’t necessarily remember what “the place” is for something you only need, say, every three years. So instead of trying to brute-force memorize it, you make it easy for your future self to deduce it.
Keeping a master list of where stuff is, is also necessary. But there’s so much stuff in the modern world, physical and digital, that after a while the lists themselves become unwieldy!
Tim Spalding, the creator of LibaryThing, also uses this advice for classifying books by subject. Here, instead of coordinating with your future self, you coordinate with other readers. The idea is not only what the “right” subject is, which works for some books (e.g., a book on algebra obviously should be classified as Mathematics). It’s also to settle edge cases by asking, “Where is a reader likely to look for this?” So for example, Dracula or Alice in Wonderland might reasonably be categorized as Fantasy, but a modern reader is probably more likely to look for them in Classics. So if you must choose one category – as a physical bookstore or library must do when it chooses what physical shelf to put the book on – then choose Classics. That will make it easier on most readers.
Cooperative game theory for the win!