OJ

An exercise recently posed in the Jenna Moran fanclub chat was to write Apocalypse World-style principles by reverse-engineering them from your own writing and GMing. So here are some of mine:

• Pretend you did it on purpose. Backfill the reasoning and logic leading up to it later. So maybe you did something on a whim, or declared some piece of lore without fully thinking it through. Retconning something that already happened ingame shakes the players' trust. Coming up with a reason why an exception happened is great for worldbuilding and makes you look like you're prepared.
• Prepare NPCs, not scenes. Don't presume that the players will act a specific way around your NPCs. They will not take your bait. If you build NPCs with motivations - even if the motivations are one or two sentences - their reactions to the PCs' actions will make interesting things happen.
• Prepare settings, not scenes. You can make places, and associate NPCs with them. But don't prepare several NPCs to have specific interactions with each other in a specific place, because then you will be tempted to force the PCs in that direction. (For example, don't make specific NPCs have a pre-scripted barfight.) Leave yourself the option to move around some of the NPCs if the PCs' attention wanders.
• Round uncertain results in the players' favor. If your system has wiggle room, or you're not entirely sure how someone or something will react, you usually want to round the result towards the players. Doing so makes them feel like they're getting away with something. Not doing so runs the risk of being seen as unfair and capricious.
• If they're paying attention to something, it's important. Insisting that something is "unimportant" will just make the players more suspicious. If the players are focusing "way too much" on some random object, stick some backstory on the object and make it have plot.
• Glue together existing pieces in a new way. True originality doesn't actually exist, so please feel free to steal fragments from other works or scenario-books. (Or even base some NPCs on actual people with the names swapped out.) But see "prepare settings, not scenes" above: trying to replicate vignettes or plots wholesale will feel forced and/or tip off players who are familiar with said works.
• The players don't already know. If you want to have a Deep Secret in the heart of your setting, trying to only give the players tiny hints will often result in them completely missing the hints. Play out all the consequences of the Deep Secret, hint very heavily at it repeatedly, everything short of actually saying it outright: this gives them a realistic chance of figuring it out, or at least of realizing that you were indeed foreshadowing things when you finally reveal the secret. (An example of this would be the Pink Diamond shenanigans in Steven Universe.)
• By definition, it's already symbolic: of the inside of your brain. Don't force symbolism or philosophical questions. If they're relevant, they will come up naturally, and have real resonance because they're likely based on real problems you ask yourself about. If they're irrelevant, trying to add them will seem preachy and out of place.