"The better you write, the higher you go in Ogilvy & Mather. People who think well, write well. Woolly-minded people write woolly memos, woolly letters, and woolly speeches.
"Good writing is not a natural gift. You have to learn to write well."
That's the preface for David Ogilvy's famously simple memo to agency staff on how to write (which Ogilvy consequently published in The Unpublished David Ogilvy—now that's marketing).
Here's a list of Ogilvy's 10 tips for writing well. The first is to "read the Roman-Raphaelson book on writing. Read it three times." He's referring to Writing That Works, by Kenneth Roman and Joel Raphaelson.