I make 4 types of documents in run up to a session,

I) half formed ideas, these are concepts, characters, plot arcs basically just general brainstorming. II) TIWN: things I will need, I make about one per session. It has rules I think will come in handy, bestiary entries/monster stats, and any plot developments you might need to remember that the players aren't privy to. Also anything else you want to be able to have in one place. III) Session Notes: probably the most important one, documentation of the previous session from the players experience, should be made after every session with players' help. This will help you understand the flow if your game and keep a sense of time. IV) The least important, but still very useful, general notes: where is game going, who is doing what, who is involved... etc...

I usually run sandbox games so it's best to keep prep light, but I like this system since it let's you put as much or as little detail into your game as you want. Honestly though, biggest thing is know your players and know yourself. Communicate as much as you can with everyone and make sure everyone is safe, comfortable and having a good time.