The Worst Mistake Made on a Dissertation Is...
I have a saying that I like to tell consulting clients, which is easier said than done, but I think are words for doctoral candidates to live by: "The only bad dissertation draft is one that isn't turned-in." The most common factor that unnecessarily slows progress on a dissertation proposal or defense is a propensity to strive for the perfect draft. As a graduate student, we all fantasized of turning-in our first draft and having our advisor, being so amazed at its brilliance, insist that you accept your PhD on the spot.
Unfortunately, reality inevitably sets-in in the form of red-line-filled draft that features more recommendations than the surgeon general and more added work than you ever imagined would be forthcoming. The feeling can be both crushing and disheartening. The good news is: the pain can be avoided, or at least reduced to a minor setback.
The key to limiting the frustration inherent in this process is: TURN YOUR DRAFT IN! Relinquish your dreams of producing a perfect draft and turn-in the draft you have. The sooner you get a draft on your advisor's desk, the sooner you can receive their valuable feedback, allowing you to spend more time integrating it and less time on details that they may (or may not) ultimately approve of.
Please don't misinterpret what I'm saying here: I'm not suggesting that you should turn-in work that is blatantly of sub-standard quality. Certainly an advisor's time is valuable and not to be wasted. However, I am suggesting that you let go of the fantasy that your first draft will be perfect. When you feel that you've made an honest effort to move your manuscript forward, turn in a draft and allow your advisor to direct you to where things can be improved. The reality is that many details that are agonized over in early drafts are made irrelevant by recommended cuts and revisions by our advisor.
As a warning, the concepts I'm discussing here will feel unnatural to many readers. In fact, most doctoral candidates are high-achieving by nature, so striving for anything less than perfection may feel uncomfortable, or even down-right wrong. However, you'll be happy you heeded this advise when you receive your advisor's feedback and know that you are moving closer to your degree and saved yourself a lot of agonizing and wasted energy along the way.
Understand that major feedback is unquestionably coming and try to get your hands on that feedback sooner, rather than later. The alternative is spending day, weeks, or even months longer on a draft, only to receive a very similar amount of feedback, with a whole lot more frustration.