Stephen King said something like "To become a better writer, write a lot and read a lot". Here's a couple of articles that discuss that idea more:

http://www.beliefnet.com/columnists/dreamgates/2011/06/read-...

http://writetodone.com/how-to-use-reading-to-become-a-better...

Beyond that, there are, of course, tons of books for writers, including some specifically for tech writers. A lot of what you'll find on the shelf at, say, Barnes & Noble is about fiction writing, but then again, to some extent you could say that good writing is good writing regardless of whether it's fiction or non-fiction. I don't claim to be a great writer, but I can share a few titles from my own bookshelf on writing:

1. The Elements of Style - Strunk & White

2. On Writing Well - William Zinsser

3. NLP For Writers - Bekki Hill

4. A Certain 'Je Ne Sais Quoi' - Chloe Rhodes

5. A Manual For Writers of Term Papers, Theses, and Dissertations - Kate L. Turabian

6. The Elements of Technical Writing - Gary Blake & Robert W. Bly

7. The Art of Scientific Writing - Ebel, Bliefert, Russey

8. Writers Digest Handbook of Magazine Article Writing - Michelle Rubert (ed.)

9. On Writing - Stephen King

10. Zen In The Art of Writing - Ray Bradbury

11. Eats, Shoots & Leaves - Lynne Truss

12. Where Do You Get Your Ideas - Fred White

13. The Complete Guide to Article Writing - Naveed Saleh

14. HBR Guide To Better Business Writing - Bryan A. Garner

15. Make A Scene - Jordan E. Rosenfeld

16. Beginnings, Middles, & Ends - Nancy Kress

I'm also a big believer in having a strong vocabulary, even though I don't try to write in a pretentious, sesquipedalian fashion very often. But sometimes it helps to have that "just right" word available. To that end, I recommend:

17. Big Words You Should Know - David Olsen, Michelle Bevilacqua and Justin Cord Hayes

18. The Words You Should Know To Sound Smart - Robert W. Bly

19. The Dictionary of Difficult Words - John Ayto

20. Rogets Thesaurus of Words For Intellectuals - David Olsen, Michelle Bevilacqua, and Justin Cord Hayes

21. The Thinker's Thesaurus - Peter E. Meltzer

22. Fiske Word Power - Fiske, Mallison, Mandell

23. The Well Spoken Thesaurus - Tom Heehler

24. They Have A Word For It - Howard Rheingold

and remember, as Aldous Huxley said "The proper study of mankind is books". And on that note, may I recommend keeping a couple of books of quotations around. A good quote can be quite useful.

25. Mark My Words - Great Quotations and the Stories Behind Them - Nigel Rees

26. The Giant Book of American Quotations - Carruth & Ehrlich

as for websites and blogs and what-not, there is a technical writing subreddit:

http://technicalwriting.reddit.com

and https://writing.reddit.com

and

https://writinghub.reddit.com

You also can't go wrong studying the essays of George Orwell

http://www.orwell.ru/library/essays/index_en

And I think that's about all I have to contribute, except to reiterate "write a lot, read a lot".

Source: I have no particular distinction as a writer, but I have managed to get a few articles published at InfoWorld: http://www.infoworld.com/author/Phil-Rhodes/ and I've written quite a few blog posts for my full-time employer, like: http://mammothdata.com/big-data-open-source-risk-managment/ and I have written most of the content on the Fogbeam company blog: http://fogbeam.blogspot.com I like to think that if I keep reading and writing, I might actually acquire some skill at it some day.

HTH, YMMV, IANAL, WTFBBQ, ETC.