Rudolf Flesch authored an essay "Preparation of Effective Reports" for Industrial Engineers in the Maynard's Handbook of Industrial Engineering,2nd Edition. He included as important four habits for effective writing.
In this essay, fluency is the topic of description.
Fluent speakers are many, but fluent writers are not many. This can be because speaking in public was much more frequent than writing for public in olden days. But now that writing opportunities are more due to the availability of blogs, there are opportunities to develop fluency in writing through blogging. The universal remedy for lack of fluency is practice. So a lack of fluency in writing can be overcome only through a steady practice in writing. Once million words are written, one becomes professional and starts producing a planned quantity of publishable prose at every sitting.
To practice writing correspondence with friends and relatives can be increased, diary can be maintained, notes can be made on various articles and then public writing on blogs and chat rooms can be attempted. With practice, one does not feel nervousness which comes when one is first writing for public and feels that public may reject his writing. Each successful article creates more confidence in a writer and increases his fluency.
How to improve writing fluency