Shivers makes one main point, but makes it well: a thesis is an idea, and a dissertation is a document that supports your thesis. This clarifies a lot of thinking about the task, to wit,
You will know what things are essential, and what things are distractions or detours. You will know when to stop writing: when you have demonstrated your thesis. If your thesis committee makes unreasonable demands of you, you will be able to tell them: “(a) My thesis, as stated, is a solid advancement of the field, and (b) I have supported my thesis. This is all I need to do to graduate; your requests are above and beyond this threshold. Cancel them and give me my degree.”
A side benefit is that it provides an unassailable defense to an entire class of attacks on your work. For example, should someone attack your work by pointing out that it does not scale, you simply reply, “You may be correct, but right or wrong, your point is irrelevant. My thesis is that ‘crossbreeding gerbils with hamsters provides an order of magnitude speedup over standard treadmill technology.’ I clearly demonstrate factors of 12-17 in my dissertation; I make no claims beyond an order of magnitude.” This is one of the benefits of focus.
In between he writes pithily about good writing.
You might also enjoy Shivers’s advice on the night before your thesis defense.