One of the best things about NaNoWriMo, or any terrifying deadline, is that it forces you to write quickly. (Hello, procrastinators.) If writing quickly is your stated goal, then you don't have time to do the number one thing that interrupts your writing flow: Think about whether what you're writing is good.
Worse than thinking about whether what your writing is good is actually trying to answer the question while you're writing. Aside from the fact that it means stopping to go back and read, evaluating your work is poison to producing it. Evaluation and generation are totally different modes of thinking, and soon enough you end up worrying about whether your writing is good before you've done the writing. And that, folks, is game over. Stopped in your tracks, paralysed by worry, sure your ideas are garbage, there you are: Not writing. So what do you do?
Write the bad draft.
Make that your goal. Do it. You'll find it's a million times easier to get started if you tell yourself, "I'm just gonna write the crappy version of this," than if there's any pressure to make what you're writing is good. Half the time, the "crappy draft" will be pretty good, anyway - once you turn off your inner critic, all of your writing energy goes toward writing rather than worrying about your output - but even truly crappy drafts can be revised to greatness. There is so much you can do in revision, from overhauling structure to drafting new paragraphs to tweaking the language of a sentence.
You just need to write a first draft first. So write a bad one.