I don't remember the original source, but in Leslie Lamport's Specifying Systems book, he quoted someone saying "Writing is nature's way of telling you how sloppy your thinking is."
Generally, if you're writing long, rambling posts, there's a good chance that your understanding of the subject that you're writing about is sloppy (not to say you don't understand it, just that your thoughts are all over the place). If you can express what you're writing in a fairly short amount of time, you probably have a relatively good mental model of what it is you're tying to say, and this is a learnable (and useful) skill.
I've found that getting into the practice of writing "notes that I will actually read in the future" has helped a lot with this.
Generally, if you're writing long, rambling posts, there's a good chance that your understanding of the subject that you're writing about is sloppy (not to say you don't understand it, just that your thoughts are all over the place). If you can express what you're writing in a fairly short amount of time, you probably have a relatively good mental model of what it is you're tying to say, and this is a learnable (and useful) skill.
I've found that getting into the practice of writing "notes that I will actually read in the future" has helped a lot with this.