The only difference between writers and aspiring writers is that writers write. -- Martin J. Smith
One of the best ways to improve your craft (besides practicing it) is find other writers and editors whose opinion you trust and get them to critique your writing. If you can gather a group of them together, swap material on a regular basis, and discuss over a good meal, even better! You are less likely to argue with them, and more likely to listen to what they say, if you are chewing your food during their critique. Just don't forget to take notes.
My third piece of advice, after writing and soliciting constructive criticism, is to read good writing—as much of it as you can find. (Hint: This is the fun part.)
Beyond that, here are a few resources I have found useful...
ARTICLES AND ESSAYS
- On Writing: A Memoir of the Craft by Stephen King
- The Elements of Style by William Strunk Jr. and E.B. White
- Zen in the Art of Writing by Ray Bradbury
- Writers' Resources by Gotham Writers' Workshop