Spoilers: It’s a 3×5 notecard.
Before you leave! Hear me out. The 3×5 notecard has been invaluable to me as a new (and day by day, less new) GM and also as a player! These babies are well worth their weight in gold usefulness wise, and they are cheap as dirt and easy to get.
Here’s the secret, you buy your 3×5 notecards, fill out an item or spell per card, get a little box to keep them in (also cheap as dirt, and can be dressed up like treasure chests) alphabetize them, and suddenly your future prep is cut in half and your improv is 100% easier.
Using 5th Edition D&D as an example:
Each time I make a character that has spells or abilities, I find that ability in the book, write it’s name and page number at the top and a brief mechanical description on the card. I usually leave out flavor text for space/time/laziness constraints. This means you have most of the key information you need, but also the ability to quickly flip to the page in the book if you need more info.
Now, obviously this means you have the card for the game/character you’re planning, but there’s more! Add it to your alphabetized box and viola! You now have it for every game and every character from this point on!
I use 3x5s for everything too, I’ll use the same process for items, abilities, spells, and monster stats. Keeping these cards around allows my games to have a faster pace, since I have less paper to read through. It allows for smoother improv, players do something clever that deserves loot? Grab a random item card from the box, give it to them (or keep a random stack next to you always). And finally the most important (in my opinion) part, have a new player that’s nervous? Give them their spells and abilities, so they aren’t as overwhelmed and can focus on having fun.
I also enjoy using a note pad and note book, note pad for quick, scrawled during session notes, and a note book (or electronic document) for organizing the information that stayed important after the session.
Then when you’re done with your day you can make a handy character relationships chart to help organize your thoughts into a easy to read and coherent…
…OK wait that’s a complicated game though, I swear the character relationship charts are incredibly clean and useful tools for GMs! All you have to do is keep your PCs under control and everything will turn out…