At some point, every serious writer is forced to sit down and conclude that there is something seriously wrong with their work. It wanders; it is pretty in some spots and horribly ugly in others. It doesn't always make sense, and is uneven in places. Even though every sentence is grammatically correct, there is something fundamentally broken about the piece.
It lacks structure.
Structure is what holds a good piece of writing together, the material reflection of the reader's psychological need for order. It is the quality that makes the best writing appear seamless, conjured whole from heaven itself. Structure is the logical mind's contribution to a creative process, and a primary difference between professional writing and amateur scribbling: a conscious decision and a learned skill.
Being in many ways the very essence of writing, structure isn't mastered overnight. But here are a few rules of thumb that can help you improve the organizational readability of your work:
Establish a logical order to your presentation.
Ignore all the popular advice to "write like you talk"; that's a misguided appeal to conversational tone usage and a shortsighted encouragement for people who are terrified to put pen to page. In order to master structure, you must learn to write deliberately and with forethought. Plan what you're going to write and how you're going to write it: don't make it up as you go along, particularly when you are writing nonfiction of any kind.
In nonfiction writing (which means anything that isn't fiction), the room for art is small. Don't set out to create art - build a sturdy framework, as a skilled attorney would build a legal argument. Make your supporting points early and establish the logical flow to consequences and conclusions. Don't loop back and make points at word 800 that you should have made at word five.
Make your points quickly - write in 300-word chunks.
That's the magic number: 300. Books are typically printed with about 300 words to a page; magazine articles will usually be structured into roughly 300-word segments. An effective press release, page of website copy or newsletter article won't run much above 300 words. Any longer and your reader will notice that something is off about your piece. Too much longer and your reader will get bored. For some reason, the human mind seems to be most comfortable reading at the 300-word length.
That does not mean that everything you write must be short, only that long pieces should be built out of short pieces put in order. If you can't make your point in 300 words or less, then you are trying to make more than one point. Simplify the whole piece: break the manuscript down into single-point segments no longer than 300 words in length, and then put your points into a logical order that builds towards your final conclusions. The final product will seem to flow with a gentle rhythm that your readers might notice, probably won't be able to identify, and so will most likely attribute it to your talents.
Try it: you'll be amazed.
Take the entire piece down to a single thought, expressed in a single sentence, and then rebuild it from the ground up.
When in doubt, strip the piece down and rebuild it from its primary components. The greatest threat to structure is diffusion; rather than trying to communicate one thing well, you end up saying lots of things badly. Good structure requires that you have a very clear idea of what you are writing, how you are doing it and why. Do one thing, and do it very well.
Set the piece aside and attempt to make your final point in a single sentence, losing as little important detail as possible. Do not use compound sentences; keep it simple and limit it to a single direct thought. If you can't do it, then you do not have a clear enough idea of what it is that you're trying to accomplish - reorganize the piece or split it into several separate ones.
An English sentence has a natural internal structure all its own. Look at your one-sentence summary and use its structure to inform yourself on how the overall piece should be structured. Once you've reduced your writing to its bare essence, you can reconstruct it on a much more solid foundation.
In the end, professional writing is all about understanding the psychological needs of the reader. If you are writing purely for your own pleasure, with no intention of ever letting anyone else read it (and what a boring life that would be), then it doesn't matter because you're not really writing: you're keeping a diary.
But if instead you want your writing to be appreciated by readers, structure is one concept that you can't live without.