It feels like the more I read about writers, the key indicator of quality prolific writing involves the focused and daily practice of writing. Sitting your butt down on a chair (or standing) and writing for a few solid hours. This practice involves strength of patience, focus, and the ability to write and sit for long periods.
Beyond this it seems like concerted focus on rewriting, editing, and improving what you write is the tail indicator for success. Someone that can sit down, put words down on paper, and fairly critique what they write and improve on it tend to be the best writers.
Fitzgerald was known for spending unheard amounts of time perfecting specific sentences, paragraphs, and short novels beyond any other writer. Some of the best movie editors will spend months if not years perfecting a cut of shots for a scene so the pacing is just right.
From what I can gather, the best way to improve your writing is to let yourself write daily, to let it flow freely and then refine that before publishing it to others.