Read "Style: Ten Lessons in Clarity and Grace" by Joseph Williams.

It is not one of those books like Strunk and White's that teaches you a set of arbitrary rules.

Rather, the author defines a set of general principles. Then he develops and illustrates those principles with good and bad examples. And talks about the exceptions to those principles.

Much of what separates really good writing from merely correct writing seems ineffable; it sounds the right note. Williams makes many of those seemingly indescribable qualities identifiable, so you can then recognize them in others' writing and learn to build them into your own.