Read this.
If you're really interested in improving your prose I would recommend:
- Learning the elements of English grammar
- Close reading of writers whom you admire
- Lots of writing on your end
- Most important to step 3, after you finish writing, take a sober and measured evaluation of what you produced. Identify what you did well and what needs work. Maybe list out your weaknesses on a piece of paper and keep them handy while writing.
An important thought on editing: the editing process is finished, not when there is nothing left to add, but when there is nothing left to take away. When you are editing your paper, with every word, every sentence, and every paragraph, you should be asking yourself, "Do I really need this to be here?"
By close reading I mean, as was said above, spending a long time breaking down the prose of your peers into its basest elements, and understanding how they do what they do. From there you can begin to develop your own style and thereby grow as a writer.
I'll get both (and Stephen King's On Writing). I'm pretty sure I'll find a PDF online or something instead of having to buy them.