Start by dissecting articles at authority sites. Strip them down to the bare minimum, and you'll see that they're all cut from the same cloth. Copyblogger.com has some decent free ebooks and articles on 'content-marketing' -- just a big word for blogging.
If you're using Wordpress, which I guess most people do, you can install Jetpack. This is a plugin with vast amounts of features. On the content side, it has a brilliant spell checker, which also checks for cliches; redundant words; passive voice and so forth.
When I outsource my writing to general writing services, I always get shit back. I know that most writers are paid per article/word, so naturally they have no interest in providing anything more than what is acceptable. If I have to spend time on rewriting something, I might as well just write it myself. My ROI for spending an hour on an article has so far been more than good. I never blog about stuff I'm not interested in anyways. I take one thing which I am interested in, i.e. investing or fitness, and twists it over to a more profitable angle.
If you are interested in improving your writing -- I mean, actually spend time on getting better -- there are written numerous books on writing and copy writing for web.