To write well, read well. Read good books, which are often, but not always, old books. If you carefully read, say, William James' Varieties of Religious Experience, you will learn something of the expository potential of the English language from a master of thought and expression. If time is short, study one of his popular essays such as "The Moral Philosopher and the Moral Life." Here is a characteristic paragraph:
But this world of ours is made on an entirely different pattern, and the casuistic question here is most tragically practical. The actually possible in this world is vastly narrower than all that is demanded; and there is always a pinch between the ideal and the actual which can only be got through by leaving part of the ideal behind. There is hardly a good which we can imagine except as competing for the possession of the same bit of space and time with some other imagined good. Every end of desire that presents itself appears exclusive of some other end of desire. Shall a man drink and smoke, or keep his nerves in condition? — he cannot do both. Shall he follow his fancy for Amelia, or for Henrietta? — both cannot be the choice of his heart. Shall he have the dear old Republican party, or a spirit of unsophistication in public affairs? — he cannot have both, etc. So that the ethical philosopher's demand for the right scale of subordination in ideals is the fruit of an altogether practical need. Some part of the ideal must be butchered, and he needs to know which part. It is a tragic situation, and no mere speculative conundrum, with which he has to deal. (The Will to Believe, Dover 1956, pp. 202-203, emphases in original)
One who can appreciate that this is good writing is well on the way to becoming a good writer. The idea is not so much to imitate as to absorb and store away large swaths of such excellent writing. It is bound to have its effect. Immersion in specimens of good writing is perhaps the only way to learn what good style is. It cannot be reduced to rules and maxims. And even if it could, there would remain the problem of the application of the rules. The application of rules requires good judgment, and one can easily appreciate that there cannot be rules of good judgment. This for the reason that the application of said rules would presuppose the very thing — good judgment — that cannot be reduced to rules. Requiring as it does good judgment, good writing cannot be taught, which is why teaching composition is even worse in point of frustration than teaching philosophy. Trying to get a student to appreciate why a certain formulation is awkward is like trying to get a nerd to understand why pocket-protectors are sartorially substandard.
But what makes James' writing good? It has a property I call muscular elegance. The elegance has to do in good measure with the cadence, which rests in part on punctuation and sentence structure. Note the use of the semi-colon and the dash. These punctuation marks are falling into disuse, but I say we should dig in our heels and resist this decadence especially since it is perpetrated by many of the very same politically correct ignoramuses who are mangling the language in other ways I won't bother to list. There is no necessity that linguistic degeneration continue. We make the culture what it is, and we get the culture or unculture we deserve.
As for the muscularity of James' muscular elegance, it comes though in his vivid examples and his use of words like 'pinch' and 'butchered.' His is a magisterial weaving of the abstract and the concrete, the universal and the particular. Bare of flab, this is writing with pith and punch. And James is no slouch on content, either.
C. S. Lewis somewhere says something to the effect that reading one's prose out loud is a way to improve it. I would add to this Nietzsche's observation that
Good prose is written only face to face with poetry. For it is an uninterrupted, well-mannered war with poetry . . . (Gay Science, Book II, Section 92, tr. Kaufmann)
A well-mannered war, a loving polemic. There is a poetic quality to the James passage quoted above, but the lovely goddess of poetry is given to understand that truth trumps beauty and that she is but a handmaiden to the ultimate dominatrix, Philosophia. Or to coin a Latin phrase, ars ancilla philosophiae.
Finally, a corollary to the point that one must read good books to become a good writer: watch your consumption of media dreck. Avoid bad writing, and when you cannot, imbibe it critically.