I used to do NaNoWriMo every year, and managed to achieve the goal of 50,000 words every single time—one year I even overshot it and got to 60k. Though I will readily admit that some of my tactics involved in doing so, especially for my first few attempts, might have been a little underhanded. 
Such tactics include:
Do not delete anything under any circumstances. It does not matter if the scene you're currently writing directly contradicts a previous scene, because you can edit it later.
Writing challenges. Among writer circles, people will often put out silly challenges, like "have a character introduce themself as Gary Lampshade," and I would do my best to work as many of those as I could into the story.
Extra long descriptions. Describe scenery in slightly too much detail. Do not be afraid to forego contractions if you are really struggling to hit your word count goals. If it is possible to spell something as two words, spell it as two words (e.g. I normally spell it as "alright," but for NaNoWriMo I always forced myself to use "all right" instead)
In extreme circumstances, sometimes you can get away with doing something really silly, like breaking the fourth wall and having an argument with your characters in-story.
Obviously you don't have to do all of those things if you don't want to, haha. You don't even have to force yourself to do 50,000 words if you don't think you can do it. But the general goal of NaNoWriMo is to get you to let go of writer perfectionism and just write something, doesn't matter if it's good or not.
