Creativity and Improvisation
One of the most difficult tricks to pull off as a GM is game prep. Doing enough that you're prepared for your game and the creativity of your players (aka out-of-left-field crazy b.s.), but not so much that you're wasted effort and time. It's not possible to be prepared for every eventuality. You are outnumbered by your players, and one of their brains will come up with something your brain did not. So you're going to have to improvise. I like the word "improvise," because it sound better and more deliberate than "scrambling." Like any skill though, this is something that gets better with practice.
The real secret is that you're already creative. You already have great ideas. we just don't always remember them. If you've ever had the best idea ever right before going to sleep only to wake up in the morning trying to remember what it was, you know what I'm talking about. So you just need a better way to catch those wayward great ideas and put them somewhere useful. Enter the mighty Index Card.
Anyone creative should have some way to capture those ideas that normally get away. There are digital tools, like Evernote, but I am a huge fan of the index card stack. This is partly because I work in a restaurant, and it's not always cool to pull out my phone to add something digitally ("I know you're ready to order, but I just need to do this real quick.") and partly because I like the tactility of the cards.
Making a Stack
Step zero: Get some index cards and binder clips. Something to write with is good, too. If you can see the receipt, this cost me a little over six dollars, half of which is because I like fancy G2 pens. If you already have something to write with, you'll spend even less.
Step one: Taking things out of the box. I got fancy and bought colored index cards to act as section dividers. Note that the smallest amount of binder clips was a dozen, which is three times what I needed.
Step two: Dividing things. One hundred index cards is going to make four stacks for me, so I split the white cards into piles of 25, and took four of each color from the colored index cards (leaving lots and lots for later stacks!)
Step three: Organize your stacks. My idea capture stack is going to be divided into Things to Do, Calls to Make, Gaming and Stories.
Step four: Attach the binder clip. Yeah, that's totally a step.
Now when you have an idea, a name for a character, a scene, a website you want to remember, you can jot it down on a card. This is your capture stack.
It's important not to leave things in your capture stack. Set yourself a time to go through this stack to process your cards. (I do this in the morning before work, so I can take care of the Calls to Make category.) For cards in my Gaming category, I take them out of the Capture Stack and put them into a stack for each game.
Using the method above, put together a stack for your game. The sections of my game stacks are People, Places, Things, and Scenes. The scene section pretty much is the plot of the adventure. A scene is going to have a Location, there are going to be People there, and the rest of the card is for notes about what's going on in the scene.
My location cards have notes about who might be there, what the place looks like, and any scene aspects might be available.
Here's an example of a person card, in this case a creature for my Masters of Umdarr game: a giant Ice Leech:
The important thing is not to include information you don't need. Ideally just enough to riff off of and improvise. For NPCs I like to note motivation and goals, and relevant stats.
Let me know how you use index cards in your GM Prep, or if you get a chance to try something like this. Hopefully capturing your ideas and processing them will help you use them more often. Don't tell anyone, though. Let them think you're smart and amazing. It's great.