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(+8)

First off, co-signing what SnowyStrix said. Principles and moves are one of your key chances to inform the MC what your game is / is about / feels like in practical terms rather than aspirational ones. Try to get a clear sense of what (variously) genre, tone, mood, arc of play, feeling, etc you're aiming for and write to communicate that. 

For principles, my take is this: different principles address different parts of MCing the game, and it's good to be clear which are for which as you're writing. Some principles tell you how to run the game (address yourself to the characters, not the players; make your move, but misdirect; sometimes, disclaim decision making). Some principles signpost the feeling/tone you should aim for (barf forth apocalyptica, look through crosshairs, respond with fuckery and intermittent rewards). Some principles tell you what kinds of things to put in the game (name everyone, make everyone human; ask provocative questions and build on the answers). 

Most good principles, to my mind, do more than one thing at once. 

Like, look through crosshairs tells you all at once that Apocalypse World is dangerous and should feel fraught and ripe with the potential of loss, and tells you that it's not your fucking job to be precious and protect people (NPCs or otherwise), and tells you when someone or something comes on screen to "consider first killing it, overthrowing it, burning it down, blowing it up, or burying it in the poisoned ground" (thanks Vincent). 

I think one of the things that makes for strong, memorable, actionable principles is that multiplicity. Come up with statements that give the GM all at once a cluster of good advice.

Being poetic or metaphorical is good, if your use of those techniques is strong and clear.  Principles should start with an active, actionable verb. And at least a couple of them should point pretty directly to the themes of / central assertions about the world made by the game (in AW for my money this is look through crosshairs & name everyone, make everyone human)

Also, principles are a key place to signal the voice of your game. There's a world of difference communicated by the difference between look through crosshairs (Apocalypse World), think dangerous (Dungeon World), show them the barrel of a gun (the Sprawl), hold a sword to their throats (To Tread the Spiral Path, my game), and make death common and make it matter (All Things Under Heaven, another of my games), even though we're all gesturing to a similar direction.

For moves: use an actionable verb, and be as straightforward and plain-spoken as you can. This probably isn't the place for poetry.

Dungeon World and Apocalypse World both have a section that's like (paraphrased) "these moves aren't jargon, they mean what they mean, just do the thing", but if we're real they both can get jargon-y. Inflict harm (as established) and announce off-screen badness feel intimidating and a little obscure; hurt them and show distant trouble do the same work and feel (at least to me) a lot more accessible as tools to reach for and immediately actionable to use.

All of this is, of course, just my opinion, but I hope it's helpful. At this point I've got like, 3 (4?) full-scale AW-adjacent games in the works and the MC stuff (agenda, principles, moves) feels both more intuitive to tackle and more rewarding to write with each new game.