One of the hallmarks of getting very good at something is that certain elements become habit. A musician doesn't really think about tuning his instrument or running through a few familiar tunes. An athlete just falls naturally into proper form. This is one of those things that's hard to measure because it's not what they do that distinguishes them - anyone can do that with a little effort - but how they go about doing so.
For storytellers of various stripes, one of the ways this manifests is as an instinct for scenes. They see something - a person, an idea, whatever - and they are struck by ways in which you might do something neat with it. Like other instincts of expertise, this is something that anyone can do with a little thought, but with experience, the possibilities tend to jump up and smack the storyteller in the face.
This, far more than any ideas about grand narratives, is the skill from storytelling that translates most strongly to GMing. The most consistent challenge that the GM faces is "How can I make this work?" and a lot of the worst habits of GMing emerge when no answer presents itself. Fiat rears its ugly head when the GM doesn't have a way to work with the current situation, and thus she forcefully changes it. Not only does that kind of stink on its own, it also usually means the GM is ignoring or dismissing an element that a player brings to bear, leaving the player that much more alienated.
Now, here's an important note: this is not just a hippy-dippy narrative problem. When you deal with hard core game-playing, one of the big appeals of RPGs is that it supports opportunities to think outside the box. When presented with a challenge, you can engage the environment in unexpected ways to achieve clever solutions. These solutions are every bit as much a player-initiated game element as the more story-oriented player who uses a bit of narrative authority to talk about an old girlfriend they left in this town. We invest a lot if energy in differentiating these different types of player ideas, but in the end, they're all ideas, and how the GM handles them says much more about the game than any other division.
The good news is that this instinct doesn't require that you be a writer or the like to practice. You get a little bit better at it every time you see things take a turn in a game and think, "Ok, how would this look if this were what this session were about?" Brainstorm ideas, kick them around, think what you would do with them. Steal plot twists and macguffins from TV, Movies and books and think about ways you could have handled them. In time, it becomes habit, and that does good things for your games, provided you commit to it. And that reveals the tricky part.
While the obvious part of the equation is being creative, the less obvious (and perhaps even more critical) part is being attentive. Most of us know what to do when an opportunity is presented to us, but the danger is that we can easily overlook when opportunities arise. If we don't make an effort of listening to our players and paying attention to when things shift, we'll miss our chance.
Practicing attentiveness is a little harder than practicing creativity, but it can be done. Every situation where you pay attention to people and what they want can help, not just games. It's not very different from being a good conversationalist (rather than merely dominating a conversation). And thankfully, it's one of the easiest things to talk to your players about after a game, because it's easy to couch the question in non-confrontational terms - "Are there any threads or events that came up in the game that you wish had been pursued?" is a pretty innocuous question, but it can reveal a lot.
So, creativity and attention: make a habit of them.
Editor's Note: I've changed my posting time from 10am eastern to 1pm eastern, just to be kinder to the west coast.