In my estimation, here are the best and most helpful tweets (or posts) for writers that I collected in 2010.
Craft & Technique
Overwriters Anonymous: for fiction writers, how to avoid dreadful prose that gets you rejected
[from Editorial Ass, a traditional book publishing editor]
Urgency vs action in your writing
[from published author Teresa Frohock, tackles issue of action-oriented openings]
7 deadly sins of prologues
[by author Kristen Lamb]
Top 10 reasons the editor doesn't love your book, or doesn't love what your friends, family, and critique group loves
A bare bones plot summary synopsis is boring. Include emotional twists of your characters.
[from Glen C. Strathy, freelance writer; some of the best how-to advice I've seen on writing a synopsis]
Ultimate Checklist for Submitting to Publishers: 10 Tests a Novel Must Pass
[from Deborah Halverson, a published author and previous editor]
Marketing & Promotion
Why Professional Writers Need a Blog. Or Not.
[from ProBlogger; good, balanced take on blogging issue]
The Psychological Principle Behind Marketing Success In a Networked World
[by author Jeff Sexton]
The secret ingredient to a powerful author platform
[by author Justine Musk]
The Artist’s Responsibility (just because you CAN publish anything, doesn't mean you should.)
[by author Simon Pulman]