I've been reading John Gardner's book On Becoming a Novelist, and he gives four tips on how to sharpen your writing skills. I thought I'd pass them along to you all. Grammar -- If you're shaky here, you'll need to work read a college freshmen level grammar book. Mr. Gardner recommends W. W. Watt's An American Rhetoric. A friend of mine, who taught freshmen comp at Penn back in the early '70's, recommends The Elements of Style. The Elements of Style is shorter and cheaper, so unless you really need help, I'd go with Strunk and White's little book.
Writing Exercises -- Gardner exhorts the novice to develop some good writing exercises to help you learn the craft. For example, plot a short story or a novel that you have already read; write a page-long sentence describing the weather, your parents, a mythical beast, or whatever. The goal with all of these is to learn how language works. But you need to be careful here. A writing exercise helps only so much. Nothing beats writing story after story after story. Nothing.
Vocabulary -- Here, Gardner takes a cautious but active approach to developing vocabulary. Cautious insofar that you need to be wary of learning only big words because then your writing will sound phony and pretentious. Active, because a good vocabulary will not develop otherwise. His suggestion is to read an abridged dictionary and make a list of relatively short and relatively common words that you do not use on a regular basis and then to try to use them as naturally as possible in your speaking and writing.
By the way, this isn't Gardner's invention. When asked about vocabulary, Hemingway told the young author to read the dictionary three times and then throw it away. There seems to be a trend here: a writer wants a good vocabulary as well as a natural vocabulary.
Gardner also gives a final bit of advise for the really serious student -- study languages. Greek, Latin, and French are the most essential. I know Latin (from my college days) and there is something to reading Virgil or Cicero in their native language.
Imitation -- Gardner stresses the importance of imitation for the novice to help the novice understand how a master of the craft uses language. You can do this by (1) reading everything you read very, very closely, and (2) reading those authors who have been universally accepted as masters of the craft -- Austen, Twain, Faulkner, Joyce, Hemingway, O'Conner, Percy, Greene, Waugh, etc. Make sure they wrote in English; translations don't help so much. With every author, you need to ask yourself, "What don't I like and why," and, "What do I like and why." Analysis is the key.
A more arduous exercises is to spend some time typing out a short story written by a master. Gardner recommends "The Dead" by James Joyce. This exercise will force you to pay extra-close attention to how a master uses the language. However, Gardner does stress that what you learn here is limited; thus, you can only do it once or twice. So make sure you choose wisely.
As you write your own stories, you should imitate what you have learned from the masters. At first, I was a bit suspicious about this piece of advice. But as I thought about it, I realized that as you try to imitate someone, you are forced to think about language and the writing process on a new level, and the deeper you get into the craft of writing, the freer you are when you write your own stuff.
I hope this helps. At any rate, I'm sure it will stir up a lot of debate. 
[This message has been edited by Balthasar (edited April 04, 2003).]