When planning a session, I like to jot down a few key events that will need rolls that are likely to come up. In call of cthulhu or delta green this will be sanity check roll events as I want to make sure these actually happen!

But regardless of the system, for one shots I definitely plan 'beats' as it's annoying to run out of time to conclude whatever's going on.

I try to keep NPC details light so they're adaptable. The important stuff is stats and skills, the rest you can improvise

I am running Star Wars (FFG/Edge). So I write down the opening crawl, which is a combination of what they did recently with some setup for the current session. I have a few more notes on important people, planets, cities, and ships for the session, and a general idea of steps they will need to go through. And last I have a few extras - uses for Dark Side points, ideas for Despair roles, and general complications if things are not flowing (or if they end steamrolling certain things so I have more content ready).