This sort of goes along with Keep Your Eye on the Ball, but Keep Your Head Down is almost as important.
When you're playing golf, if you're worried about the other stuff around you, it's MUCH harder to play well. Don't look at the guy next to you, don't look at the bird that just flew by, don't look at the pretty lights.
Keep Your Head Down. And hit the ball.
And if you're a writer, you need to do the same. Don't get distracted by twitter, don't get bogged down by the market or changes in the industry, don't look at the pretty lights. Keep Your Head Down. And write the book.