Vigorous writing is concise. A sentence should contain no unnecessary words, a paragraph no unnecessary sentences, for the same reason that a drawing should have no unnecessary lines and a machine no unnecessary parts. This requires not that the writer make all his sentences short, or that he avoid all detail and treat his subjects only in outline, but that every word tell.
Learn to write vigorously by writing often, editing until no unnecessary words remain. Always be truthful and kind. Strive to publish only what is useful to others, and to read only what is in your best interest.