1. Read great writing. If you want to write better nonfiction, read the masters of nonfiction. Michael Lewis. Tracy Kidder. Atul Gawande. Joseph Mitchell. Jon Krakauer. Christopher Hitchens. Tom Junod. If you want to write great fiction, read Murakami, Marquez, Palahniuk, Eggers, Hosseni, Oates, Atwood. (Shameless plug here, you can find stories by these folk at Byliner: http://www.byliner.com )
2. Read about writing. Some worthy books on this:
On Writing: A Memoir of the Craft, by Stephen King (really!)
The Practice of Writing, by David Lodge
The Writing Life, by Annie Dillard
Bird by Bird: Some Instructions on Writing and Life, by Anne Lamott
Good Prose: The Art of Nonfiction, by Tracy Kidder and Richard Todd
To Show and to Tell: The Craft of Literary Nonfiction, by Phillip Lopate
3. As already mentioned upthread, practice. Write something every day.
4. If you're not confident in your writing, apply your MVP lessons. Write short sentences. Use as few words as possible. Rid your sentences of adjectives and adverbs. Study your verb choices. Ditch the complex punctuation and dependent clauses. Ask yourself, What's the least amount of "writing" necessary to convey this idea or image?
5. Have fun.