I was commenting on a writer's sample yestrday, and I referred him to some advice from a book I have found extremely useful, so I thought I would mention it here.
The book is called Writing Tools: 50 Essential Strategies for Every Writer by Roy Peter Clark, and I would recommend it as much for the established writer as the novice.
I came to this book via Roy's audio programmes on iTunes U (which are also excellent, and free to download). I had never heard of him before, but was so impressed by the extracts that I bought the book - which is even better. Very accessible, yet not at all superficial, each tool is illustrated by hit-the-mark examples. This is a practical guide that will improve any writer's work, at any level or genre - I know it has improved mine.
If you want to know more about Roy, his book and his podcasts, your best bet is to start with Roy's site at the Poynter Institute.