I can't really pick just one article that has been the most helpful because everything Chuck Sambuchino writes is completely useful--scratch that, essential--for a working writer. Do yourself a favor and add this to your blog roll, reader, whatever. It is a must read, with takeaways that can be applied immediately to improve your writing life, routine, and publishing chances.
Here is just a smattering of some of his latest posts: 10 Tips on Guest Blogging and Blog Tours, 7 Best Practices for Building an Online Platform, and one of a recurring and extremely helpful series, New Agent Alerts.
He also published a book, How to Survive a Garden Gnome Attack, which is high on my to-read list.