I've been listening to the excellent "Moonwalking with Einstein" audio book. At one point, the author Joshua Foer, talks with the expert expert at Florida State University.
Basically, the big brains at FSU has found that we generally learn a skill consciously with a great deal of thought. At some point, the skill becomes largely unconscious, and we can do it without thinking about it. Driving a car is a great example of that. Most of us can drive to work and not even remember how we got there.
When a skill becomes unconscious, people tend to hit a plateau with development. In order to go past this, they have to pay close attention to what they are doing and get feedback. In other words, you need to drench the skill from the depths of the unconscious and back into the light to keep improving. Practice, they say, should be difficult.
I think the same applies to writing. It is easy to let the words form on page without much thought, once we've attained a level of skill. But the important thing, according to these experts, is to continually pay close attention to the prose and keep trying to improve it.
As always, I'll experiment and report back how it goes.