There is a scene from the movie Desperado that has stuck in my mind for years. Near the beginning of the film, Antonio Banderas character instructs a small boy on how to play a guitar. He tells the boy how (obviously) there are two hands involved in playing a guitar: the hand that strums the strings and makes the sound, and the hand that holds the strings and makes the chord. Antonio motions to the chord hand and tells the boy, "Forget about this hand," and instructs him to concentrate only on strumming. This scene reminded me of how my dad (who plays the guitar) told me to actually do the opposite: forget about the strumming hand and concentrate on the chord hand. In either case, the lesson is the same: there are two parts involved in making this art, and in order to do this art correctly, you must concentrate on one part at a time.
The same is true with the art of writing a story (and even writing in general). There are two parts to writing a story. One part is writing the story, i.e., getting the whole amalgamated menagerie of images, people, places, and events out of your head and onto paper. The other part is shaping the story, i.e., honing what you've written into a fine piece of literature. The first part is to write; the second part is to write well. The problem most people have in trying to write a story is that they try to do both of these parts at the same time. While trying to pour forth their soul through ink and paper, they keep distracting themselves with thoughts of using images, shaping themes, developing characters, and so forth. This can lead to a story being grounded before it ever truly takes off.
I speak from experience here. Many of my latest stories never seemed to go anywhere because the whole time I was writing them I was constantly saying, "What does that mean?" "How is the theme developed?" "What is the purpose of that character(s)? that place(s)? that event(s)? that image(s)?" My story writing was stagnant because of this, until I realized that those questions were to be asked when you are shaping the story, not writing it, and that shaping and writing were two totally different parts of a story's creation. Once I grasped that, I sat down and pounded out a new story, and the results were immediate and amazing. There was a freedom and flow to the whole process that had been absent before, and I found myself actually desiring to write on it more and more as the days went by. Compared to my old story writing style, the difference was like night and day.
The secret to avoiding the unfortunate calamity of grounded (and therefore lost) stories is the secret of guitar playing: forget about this hand, i.e., forget about the "shaping" part and concentrate solely on the "writing" part. However, unlike guitar playing (where you can forget and focus on either hand), in story writing, you write the story first and shape second. Without the written story, then there is nothing to shape. Try and shape while you write, and you will ground out and lose your story. Write it and then shape it, however, and it will come as natural as breathing.
Such concentration is truly a discipline. You may find (as I did) that you are constantly battling your own mind as it furiously tries to analyze and categorize your story in the midst of writing it. Such inclinations must be pushed under if your story is to survive, and that will take some effort. At all cost, while you write, you must put all "shaping" thoughts out of your mind. Just write the stupid thing; get it out of your head and on paper. Do not worry about characters, settings, events, and images while you write; just write. The "analyzing" and "worrying" part will come later; right now, what matters is that the story is merely written. Later on, you will concentrate on writing it well.