One of the first things I learned at the first conference I attended was this: good writers keep improving. Every published author that I spoke to at that conference had something to say about it.
One best-selling author said every book in his series gets better. "If you can get through the first one, you'll love the rest."
Another kept trying to convince me to buy her latest book instead of starting at the beginning.
Consistently, every one of them thought their most recent work was better than their first.
I think this is the most important rule of writing. Keep learning. Keep improving. It is a major challenge to strive to make each paragraph better than the last--but that is why we do re-writes and edits. I know that by the time I finish a book, I've improved my skill from when I started it. So I go back and make it all as shiny as the end.
Writers improve in different ways. Some challenge themselves and do writing exercises. Others read--a LOT. Still others have read every book on writing ever printed. I do a combination of the three, and I think it works for me.
What do you do? How do you improve?