I'm primarily playing Pathfinder 2 these days, so I make a lot of use out of digital resources like Pathbuilder and pf2easy.com. I also lean pretty heavily on Trilium for session prep and notes, but I'm a chronic over-preparer and struggle with improvising the world on the spot.

I use a lot of roll tables during prep. I have several of the Gamemaster's Guide to ____ books on my shelf that I pull out whenever I'm wireframing a dungeon, and I have a lot of the Raging Swan pre-built settlements that I leaf through and drop into my world when the players need a town to come across.

I also find myself turning to GM advice books every few months, just to skim over things. Right now, So You Want to be a Game Master, Robin's Laws of Good Game Mastering, and Return of the Lazy Dungeon Master (Eh? Ehhhhh?) seem to be the ones that end up on my desk. I can safely assure you, one of the advice sinks in, but I usually have my best streaks of sessions after reviewing things like these.

I also find myself returning to modules from 3.x. They were the ones I first played, and they're (mostly) pretty easy to convert to PF2. We're currently just wrapping up Forge of Fury -- or we will, once I finish converting the Allip.