The writing-up process was the most tortuous and least enjoyable academic task I have ever had to undertake. Even though I was nominally writing-up for about 9 months, most of the words got written in the final month and a bit. It was a truly horrible process: procrastination well and truly took hold - in fact, there were multiple occasions when my research on procrastination itself seemed so much more important than my thesis. Since my PhD research was essentially an individual effort (I had a supervisor of course, luckily a particularly good one, but he moved to another university while I was writing-up), I didn't have project colleagues working on similar things who I could discuss things with, or bounce ideas off of. I believe that the process would have been less daunting and painful if I had been able to do this. In my last month, there was another PhD student writing his thesis too, but on a completely different topic. This was a great help - for motivation and company: seeing someone else go through the same thing at the same time helped to put it all into perspective.
The second thing that really drove me to complete the writing process was the presence of a hard deadline. Well, actually two hard deadlines. The first was my registration period as a PhD student drawing to a close. I know extensions can usually be acquired, but it's a fairly good deadline to work to :-) More importantly for me though was the promise of a waiting postdoc position. I started the week after I submitted my thesis, and still had to move across the country - so the sooner I submitted the better. It's quite a motivator.
These two things made me finish on time, but the way it turned out probably wasn't the best way of doing it. I didn't leave myself a huge amount of time for proof-reading, even though the thesis said what I wanted it to say. But then hindsight is a wonderful thing... I'm just happy to have gotten this far :-)