LWers often wonder how to write a good post. Many great tips and resources have been suggested already, mostly on the style and substance, not on the post structure. This is an attempt to fill some of the void using a construction toy metaphor: nested state-explain-summarize blocks.
First, there is a convenient and versatile building block of non-fiction writing, often called the inverted pyramid in the news business, though I think of it as more of a diamond when applied to technical writing in general:
I have alluded to this previously in the context of an elevator pitch. It is how most news stories are structured, and it follows the rule of three: the main point is first stated, then explained in some detail, then summarized. This is similar to how an oral presentation ought to be structured.
If any of these three gets too large, it can, in turn, be constructed of smaller diamonds. These building blocks can be combined and/or nested to build a complete post. If you want to make several points, it is a good idea to dedicate a diamond to each, and nest them into a larger one that provides the context, states the points at the top and summarizes them at the end:
Why repeat the same point three times? I am no evolutionary psychologist or a cognitive scientist, but my feeling is that it is because people have to be primed to accept new information (hence the first two times). The third time is to make sure they retain what you've told them. In any case, what I tell you three times is true tends to work.
I am not pushing this format as the one true way of writing a post, just as a reliable way to get your point across. There are many ways of constructing short articles, this is a good place to learn one of them. If you think that a short article format is too restrictive, then maybe the LW Discussion section is not the best venue for what you are writing. Consider starting your own blog (or writing a Wikipedia article) and linking to it. That said, I am yet to see a long non-fiction LW post (likely including this one) that cannot be improved by some ruthless trimming.
The "non-fiction" qualifier is important: it is entirely possible to engage the reader using a fictional narrative. Eliezer does it quite successfully in the sequences, even in the long posts. However, this is an art, not a craft, and one would do well to master the craft first. My own attempt to post a bit of fiction here failed miserably.
Again, this [meta]post is about the post structure: nested state-explain-summarize blocks (and the post itself is deliberately structured this way). I did not try to address the issue of content, partly because there has been so much said about it already, and partly because it would make this post too long to hold your attention.
I hope to improve this post based on the feedback, provided there is enough interest.