Thanks. I like the Tolstoy reference above (every unhappy document is unhappy in its own way, etc) and think that this comment highlights the mechanism behind that: that there many different ways in which you can write badly: flawed arguments, poor language, bad structure, and so on.

I think that the best way to improve is detailed feedback. You can learn a fair amount from style books, but only so much, I would guess. Lots of the time, you don't see what mistakes you are making, and need someone else to point them out.

It's important that this feedback is precise: that it tells you exactly what you do wrong and what you could do better, on a sentence by sentence level, as it were. General and vague feedback is not at all as useful as it doesn't tell you what to do in order to improve. I like Christian's proposal below of a writing group where such feedback could be given.

For what it's worth I think you're already a very good writer, but of course everyone could improve. Including Orwell.

Regarding rhetorics I personally prefer texts that don't include too many rhetorical devices such as personal stories, fictive dialogues, and so on, but which instead present the heart of the matter in a precise, structured, and non-roundabout way. Tastes differ here, however.