From none other than C.S. Lewis. I came across this list several years ago, in a letter to a schoolgirl in America, and it continues to delight and instruct.
(1) Turn off the Radio.
(2) Read all the good books you can, and avoid nearly all magazines.
(3) Always write (and read) with the ear, not the eye. You sh[ou]d. hear every sentence you write as if it was being read aloud or spoken. If it does not sound nice, try again.
(4) Write about what really interests you, whether it is real things or imaginary things, and nothing else. (Notice this means that if you are only interested in writing you will never be a writer, because you will have nothing to write about. . . .)
(5) Take great pains to be clear. Remember that though you start by knowing what you mean, the reader doesn't, and a single ill-chosen word may lead him to a total misunderstanding. In a story it is terribly easy just to forget that you have not told the reader something that he wants to know - the whole picture is so clear in your mind that you forget that it isn't the same in his.
(6) When you give up a bit of work don't (unless it is hopelessly bad) throw it away. Put it in a drawer. It may come in useful later. Much of my best work, or what I think my best, is the re-writing of things begun and abandoned years earlier.
(7) Don't use a typewriter. The noise will destroy your sense of rhythm, which still needs years of training.
(8) Be sure you know the meaning (or meanings) of every word you use.