We’ve been reading for so long that it’s a very internal process. For example, while I’m reading I might not realize that I’m struggling to read a long string of prepositional phrases; I might just have the vague feeling that it’s difficult. Fortunately, with conscious effort, it becomes easier and easier to spot in my own writing. But it’s very difficult to get used to.
The main thing I wanted to mention in writing is the idea of visual frame of reference. Suppose you’re reading a book and you “lose your place”. It could be that you looked away from the page for a few seconds. Or it could be that you’re naturally moving from the end of the current line to the beginning of the next line. When all of the lines on the page look similar, it takes time — when I get lost, I usually have to scan for keywords to find my place. As a reader, you may not even really be thinking about the process of losing your place and finding it again.
As an author, the idea of visual frame of reference is important. If you give your readers easier landmarks on every page, re-acquiring a lost position is much easier. There are many ways to provide landmarks, but the most basic is to use paragraph breaks. I find it much easier to spot paragraph breaks when there is extra paragraph spacing (such as this text). Indentation is good, but it typically takes longer for me to rely on indentation alone (especially in double-spaced documents).
In general, anything that you can see while blurring your vision will work. So boldface words can serve as landmarks too. Headings, tables, figures, and equations all provide visual frame of reference. Side-notes in the margins may provide a good reference, but I rarely see documents with them. Sometimes an em-dash can also provide frame of reference.
The idea of frame of reference extends to the whole document as well. In the case of research papers, most are short anyway (<10p) so it doesn’t matter too much. The main goal here is to get each page to look different from far away.
In a long document like a book, tables, figures, and equations help give visual landmarks when you’re flipping through the pages, reducing the amount of time to find a specific sentence that you vaguely remember. Also, books can have frequent section breaks too. In general, you don’t want to over-use section breaks, but it’s much easier to read a document with frequent, detailed headings compared to a document with few headings. (Though I’m mostly talking about technical papers/books; headings might be disruptive for novels.)
That leads me to some popular layouts — many conferences use a two-column layout rather than a single-column layout. I can’t speak for others, but I lose my place less often in two-column format. In single-column format, I have to concentrate for every line wrap.
Similarly, try out the Chrome extension Readability Redux or the cross-browser bookmarklet Readability and play with the margin width. I’ve found that it’s much easier to read with medium margins compared to small or extra small margins, partly because I spend less time finding the next line.
There are many aspects of a good technical document, but it’s easy to focus on either the writing style or the content. In this writeup, I focused on the visual aspect of writing, specifically how visual landmarks make it easier to read by reducing the amount of wasted time — time spent finding the next line or time reacquiring your position on the page.
It can make an actual difference on reading in a few ways. If the reader spends less time for menial tasks like keeping their position, they’ll probably get further in your document before taking a break (especially for journal articles). That means that more of the document will be fresh in their mind, and it’ll be easier to interpret.
It can also make reading a long sentence easier, for similar reasons (you still remember the structure of the sentence).