As people say "practice". But one tip that I found helped me when I was (trying!) to write a book, was to set aside 15 minutes a day to write something - ANYTHING. Just let your pen/fingers channel the flow of what's at the top of your mind. And because there's no pressure in your mind that "you must write something good", it will make it a lot easier. It's quite a painless way of getting words onto a page, and makes the "practice" part less of a chore.
Now a lot of the time, what you write won't necessarily be useful for a blog, but after a while you should find that actually you can turn a lot of your random thoughts into more focused and topical articles.
I will get back to my book maybe, some day...