Everyone knows that I love a post-it note. They are my number one GMing resource. They stop me from diving in too deep, writing too much and getting bogged down in prep that may never happen.
When I am thinking about a game I will create post-its of key NPCs I think would be cool, and the same for locations, and even bad guys. These could be Colonel K, my quest-giver, Stilletto the villain’s right-hand hoodlum, and Baron Greenback the arch-villain of the campaign. I could have a location in Baker Street as a starting point and create others on the fly as needed.
None of these close many doors to how my game will evolve once the oracle takes over. Our prep should lead us up to the point where the game starts. We then play to find out what happens.
If you try and prep what happens after the game started, such as a planned final showdown, you are forcing a situation onto a game that may not fit.
The less you have prepped, the bigger the spaces between these hardpoints. We call this the creative void. You are free to fill the void with your own creations without it hitting any established facts.
Some solo games will be over in a few sessions, some may run for years, but they all need some void to fill.
You don’t need to use sticky notes or post-its, or index cards or even pages of a small notebook, it is more about the intention to keep your preparations limited, that is important.