A couple of years ago, then Pixar storyboard artist Emily Coats tweeted pieces of advice on making stories that she had picked up from working with Pixar, which were later compiled in several places on the internet, such as this io9 post. Later, Dino Ignacio created image macros of the individual rules which I am using in these posts.
Rule #11: “Putting it on paper lets you start fixing it. If it stays in your head, a perfect idea, you’ll never share it with anyone.”
Take notes on your story ahead of time. I know, I know, the players are going to muck things up and you’re going to have to improvise it all over the place, but you still need a starting point. You need to write down the villain’s master plan from start to finish so you can see if there are any glaring issues with it. You need to write down the tentative course of events for your game, because your mind has a way of glossing over plot holes and the best preventative against that is setting it all down in black and white. Design NPCs ahead of time and see if they’re actually interesting before you throw them at the players for the sake of the plot. You can keep some details vague in order to keep their usage flexible, but set down a working skeleton somewhere so you can check all the joints and bones.
Take it a step further: That game you’ve dreamed about running but don’t have the group to run it for? Write it down and start poking at it. Having it written somewhere might give you that extra push to go hunting for a group to show it to. It’ll make it easier to pitch the game to the group when you find it if it’s not some vague yet wonderful idea in your head. Don’t get too attached; your group might not be interested in it right now, or they might push it somewhere you don’t want to go. The advantage of doing this is that you can start fixing problems with your dream game before you’ve started running it with your group. Otherwise you might sell a group on this game, launch it, and suddenly realize that all those great ideas you had in your head don’t actually fit together like you thought they would. That’s poison to keeping a game running, because all your enthusiasm will drain away as the work required to turn the game into something good doubles.
So start writing things down. Take some ideas floating in your head and flesh them out a little. Stick them in a scrapbook until you find out where they fit.