I think of writing as a creative, right-brain activity, whereas editing is more analytical.
There are several different levels of editing and I've blogged about my process over a few posts, from the big picture (and here), and details and problems.
Editing is a continual improvement kind of activity, and I find that I approach editing non-fiction in a different way than fiction. However, there is one area that is still the same - fooling my brain about what I'm viewing.
Even with a shorter project you still get used to seeing the same file day after day. If you're like me you have a page setup that will be something like this - A4 page, 2.4cm (1") margins, double-spaced, and your favourite font.
I do some initial editing tasks on the computer, for example, a list of words for the Find and Replace function, but nothing beats editing on hard copy. Before I print it out I change the font type and size, and often the margins as well, all in an effort to make my brain think it's reading something new. This is also the reason for leaving some time between finishing the writing and starting to edit.
After reading through Lives Interrupted several (it felt like several hundred) times, I found that even changing the font and margins wasn't really fooling me enough, and so I changed the setup to resemble a print book. That was an interesting experience. I discovered a few sections that looked fine double-spaced on an A4 page, but were just one or two paragraphs on a book-size page, and therefore an indication of places where I maybe needed to break up the narrative with some dialogue.
I've also found using my Kindle another excellent way of looking at work in a different way. I see mistakes that I hadn't caught in earlier readings. It also means I don't have to carry around a large pile paper.