Normally, I really don't enjoy "how to improve your writing" articles. I prefer articles on finding inspiration, finding a good editor, an agent, etc. The How To articles are often bland and general without any really valuable advice. It's just too general to be that helpful. I prefer one on one advice based on specific pieces of my writing. However, I did find an article on the Writer's Digest website that is positively loaded with great advice. Even if you only take one pointer you further ahead than when you started, so have a look.
25 Ways to Improve Your Writing in 30 Minutes a Day