Edit: Grouchymusicologist has already covered silly grammar-nazism, passives, and Strunk and White, complete with the Languagelog links I was looking for.
\25. Write like you talk. When possible, use small, old, Germanic words.
I think this one should be deleted. The first sentence of it is wrong as written, but the idea behind it is expressed clearly and sufficiently in #26 anyway. People do not talk in grammatical, complete sentences.
As for the second half, do you really look up etymologies as you write? I have only the vaguest sense of the origins of the vast majority of words in English, and this despite taking 5 years of French in school. This advice doesn't look like it was actually meant to be followed in any practical sense, and I would need some convincing that it's even a good idea.
\14. Almost always list things in threes, in ascending order of the word length of the list item.
This advice seems similarly quite arbitrary and unmotivated.
You don't need to look up etymology to have a feeling for the sources of words. In general, the Germanic words are shorter, seem less academic, and have a lower proportion of vowels. Of course, it's possible to overdo it.
The topics of rationality and existential risk reduction need their own Richard Dawkins. Their own Darwin. Their own Voltaire.
Rhetoric moves minds.
Students and masochists aside, people read only what is exciting. So: Want to make an impact? Be exciting. You must be heard before you can turn heads in the right direction.
Thus, I've decided to try harder and actually put effort into the quality of my writing instead of just cranking stuff out quickly so I can fill in inferential gaps and get to the cutting edge of the research subjects I care about.
That's why I asked LWers for their picks of best nonfiction writing on Less Wrong.
It's also why I've been reading lots of good science writing, focusing on those who manage to be exciting while covering fairly complex subjects: Dawkins, Sagan, Gleick, Zimmer, Shermer, Ramachandran, Roach, Sacks, Hawking, Greene, Hofstadter, Penrose, Wilson, Feynman, Kaku, Gould, Bryson, Pinker, Kurzban, and others.
I've also been re-reading lots of books and articles on how to write well: Keys to Great Writing, Style: Lessons in Clarity and Grace, Elements of Style, On Writing Well, The Classic Guide to Better Writing, The Book on Writing, Telling True Stories, Writing Tools, Ideas into Words, The Chicago Guide to Communicating Science, A Field Guide for Science Writers, Six Rules for Rewriting, Writing, Briefly, and Singularity Writing Advice. (Conversations with Eliezer also helped.)
I don't know if I can become the Voltaire of rationality and existential risk reduction, but it seems worth a shot. Every improvement in writing style is beneficial even if my starry goal is never met. Also, it appears I produce better writing without really trying than most people produce with trying. (If you've ever had to grade essays by honors English seniors, you'll know what I mean.) I expect to gain more by striving where I already excel than by pushing where I have little natural talent.
(I won't try to write everything well. Sometimes I should just crank things out. To be honest, I didn't spend much time optimizing this post.)
My other hope is that a few other writers decide they would like to be the Voltaire of rationality and/or existential risk reduction. May this post be useful to them. It's a list of recommendations on writing style pulled from many sources, in no particular order.
And, just one piece of process advice. Do not apply any of these rules while drafting. Instead, write down whatever horrible shit comes out of you and do it quickly. Then revise, revise, revise.
Now: What are your favorite pieces of writing advice?