tl;dr: Divide and conquer. – aeismail♦
The Balloon Analogy
I used to hate writing with a passion. Trying to squeeze a 5 page paper out of me was like trying to juice a rock.
Eventually, though, I figured out the root cause for my procrastination.
The biggest sense of daunting I found when writing a paper is that you start with nothing, and have to end up with a significant something.
So, let's look at what you have set up. You create a new Word/TeX document, and it's there. Page 1 of 1. A blank page with that blinking cursor.
Pretty intimidating and most likely the reason why we push it off till tomorrow until it's due next week and then we've wasted forever. This is basically like us trying to inflate a big weather balloon. You do a little work, and it doesn't seem inflated at all.
I eventually got over this intimidation by reducing the size of the problem to its individual parts. Imagine trying to fill up a dozen individual party balloons with the same amount of air as the weather balloon. Each bit of inflating goes a long way, and a fully inflated mylar balloon is much easier to achieve.
Transforming one Big Balloon into a Bunch of Smaller Balloons
A strategy that we all learned in K-12 is outlining, which actually helps more now than it did then. Now, it doesn't have to be a formal outline. However, the idea behind the outline is that it can transform into a full-sized paper much more easily than a blank slate, for little actual effort in.
Stub out each individual section of your paper. If you have a glossary, add that. If you have a bunch of subsections, create them, and add the titles. Don't be afraid to dedicate an entire page to a chapter with no content, if you feel that eventually you will need it.
For basically 12 or so lines of typing, you've turned a single blank sheet into perhaps 8 or so pages with a rudimentary summary of what you're going to talk about. Filling out a single page or two is much less daunting than that previous 20-page paper we were fretting about before. This dividing and conquering of pages has turned 1 very large paper into 8 relatively easy to populate short papers.
Inflating the Balloons
After that, pick a section, any section, that you want to write. It doesn't have to be the introduction if you're drawing a blank. That's perfectly normal. I rewrote my introduction all the way to the end of my paper because the rest of the paper was evolving to include additional topics. Some sections needed more meat or could be expanded, others could be shortened or removed.
Now, type away at the keyboard on the section you've assigned to yourself until you fill it up or have run out of ideas. Wanted to write Chapter 5: Potatoes in Artwork? Go for it! Chapters 1 to 4 can wait until you're sufficiently inspired to do them, or have enough information from Chapter 5 to help give substance to the other chapters.
When you're done with the small subsection you're working on, you should evaluate whether you feel you can do more. Sometimes, after doing a relatively easy section, I enjoy tackling a somewhat harder piece and getting that out of the way. Other times, I'll need a break, and take one.
Basically, my strategy is that one shouldn't plan to, or even reasonably expect to, write the paper front to back.
Breaking the paper into parts, giving them sufficient weight so that you can see the progress as you go, and slowly but steadily inflate and become a full paper.
Author's Note: Yeah, for some reason now I write really long posts even when I don't mean to.