In poetry, rhythm may be complicated, but it is regular and stylized enough to enable almost anyone to learn to recognize it and classify it. Rhythm in prose is very nearly as important, but because it cannot be too regular without being distracting, prose rhythm is so extremely complex that no one has ever yet succeeded in developing a system that will codify and explain it. Yet every man who has a sensitive ear and reads widely is aware that some prose halts and stutters and stumbles, not because the writer has been unclear or unconventional, but simply because some writers never seem to realize when they have too many syllables or too few, too many stresses in a bunch or too long a run of words in which no particular stresses appear. …
We might say of prose rhythm what Louis Armstrong said of jazz: “If you gotta ask what it is, you’ll never get to know.”
Yet many people who could not play a note on a trumpet can recognize inept rhythms in a trumpet solo; and many writers who permit themselves to produce nonrhythmic prose could recognize their own ineptitude if they read their sentences aloud and really listened to what they were saying. Most ideas can be expressed in many ways in English – in one-syllabled words or in longer words, in shorter or longer phrases, in clauses of greater or lesser complexity. If a sentence is so regular that it falls into sing-song, if it too noticeably repeats stress patterns, if it halts in the wrong places, if it spills over beyond the point at which it should have paused, try other combinations until the rhythm seems appropriate.
– Robert Hamilton Moore, Effective Writing, 1955
People talk of having an ear for music; there is also an ear for writing. And this advice is not only for individual sentences, but for the entire narrative. Short sentences have to be balanced against long ones, complex formulations against simple ones.