The bulk of my e-mail lately has been from people seeking advice on how to improve their writing. It puts me in an awkward position since I am not a successful writer and quality of writing greatly depends on what type of writing you want to do. A journalist is not the person from whom to seek advice if you are an aspiring poet.
In fact, there are many better writers among our blogging circles. For one, Phillip Johnson, who edits for a living, could probably write 10 posts of significance on the subject. His writing is clean and forceful -- a pleasure to read, whether I agree with his opinion or not. I have learned a lot about technique by reading his blog.
That said, I will offer what I think is important here:
1) Read! I cannot emphasize this enough. Read books -- fiction and non-fiction -- magazines, newspapers, essays, short stories, poems, even blogs. The more you are exposed to, the better versed you will be in language. It also helps if you find writers you enjoy and consume every thing they've ever written. In terms of style and rhythm, I am partial to Douglas Coupland. Since he writes novels, magazines, and poems, I have a variety from which to study.
2) Copy! I am not referring to plagiarism. Not at all. But a private exercise of copying a section or a chapter from your favorite book can be helpful in training your mind and muscles to recognize how language should flow. I spent much time doing this with Hemingway, and my writing style has been greatly influenced by it. Thankfully, my personal life hasn't.
3) Write what you mean. I don't know why the human brain makes things more difficult than it needs to be. Maybe it's pride. However, I've found more people struggle with clarity than anything. Part of the problem is inexperienced writers haven't learned how to think things out, to think as they write, to always think critically. The last part is especially important. Writing is argumentation, and you need to be looking at holes in your argument to write effectively. The other part is people spend too much trying to adorn their writing with flowery language. If conversational language works, stick with that. I think you'll find the best writers use few big dollar words, and they spend much more time on rhythm -- varied lengths of phonetics, sentences, and paragraphs. The human brain is much more concerned with rhythm of language than anything.
4) Edit! I edit as I go, which leads to a different set of problems (if you change one crucial part of the sentence, don't forget to change the rest). I edit multiple times when I'm finished. What I don't do is edit before I begin. I get something on the page, no matter how disappointed I am with the content. It's so much easier to write well when you are editing than when you are starting from scratch; it's easier to improve something than to write brilliantly out of the gate. If you wait for inspiration, it may never come, but you may discover it in the process of re-writing lesser language. A commitment to reading out loud will also allow you to "hear" the language and take the role of the reader. If reading it causes your tongue to trip or your mind to anguish over what you mean, imagine how much more difficult it will be for your reader.
5) Delete and start over. If you go through all these steps and you're still not satisfied, don't be afraid to trash it and start over. Just make sure you do it while all your frustrations and complaints are fresh in your mind. This will make the second draft more fluid and your argumentation will crystallize. You may find yourself making the same mistakes in argumentation. Chances are these aren't mistakes, you just haven't worded them properly. Identify these the second time through and dwell on them ... or write several alternative wordings to see if you can better understand what you are trying to say.
I am far from an expert. I am published (many times over), but newspaper writing is formulaic. There aren't many stories I would hail as fine examples of my work. Having gone through all this process on a daily basis, though, I am confident my best work is ahead of me ... now if I can just get the right words on the page.