Michelle, I remember some writer (whose identity I do not recall) taught himself how to write by regularly copying pages and pages from books of authors he/she admired. Doing so helped the person get into the habit of writing the way said authors did, and the benefits eventually stayed with them even when they were writing impromptu (as opposed to copying from a book page).
Also, if you read a lot (assuming the quantity to quality ratio is decent), chances are that you have a better instinctive ability to assess the quality of a given sentence or passage than you think.
In my writing--whether it's a poem, a story, a letter, or a forum post--I cannot get it very good or fully coherent and concise the first time around (unless, of course, I patiently plan it out in my head in advance). So I go back and edit, until I stop feeling that the sentences I am reading are messy/inelegant. (In case you are wandering, I infrequently do that with online forum posts to the extent I would with more formal writing... so don't be scared.)
I found it also helped me to read books about writing that address a broad range of issues, and thereby suggest ways to improve most every aspect of your writing/composition. In particular, I recommend the Chicago Manual of Style--which is presently offering a free 30 day trial online:
http://www.chicagomanualofstyle.org/home.html
Hmmm... I wonder if they have it as an eBook--I might just have to buy that myself.
The paper book is a bit too heavy to lug around, though I have before.
Well... these are all the ideas I have. Although I'm afraid none of them are quick-fixes. And, please, do forgive me for not taking additional time to tighten up my post a bit more.
- Ahi