I do minimal preparation, but I do some. I don’t like starting too much in advance though. One week ahead is about as far ahead as I like to think about the actual story.
The first year, I actually decided to do NaNo at about 8PM Oct 31. I spent most of Nov 1 brainstorming using a 300-word prompted exercise I’d done earlier as a jumping-off point, then started writing on the 2nd. Midway through the month I took another brainstorm day, and finished pretty much on schedule. (Note that while you can’t use anything previously written in your NaNo, you can use something previously written as an idea source.)
The second time I won, I think I did a bit of loose planning about the story, and again took a brainstorm day partway through.
I have found that I can’t really plan the story without knowing the characters, and I don’t know the characters until I start writing, so I like to start with a bit of randomness, let the characters wander around for a couple of days while I figure out who they are, then figure out what sort of story would fit them. Last year (my third win) I got enough about the characters by the end of the third day’s writing and the story started to get on track. Or rather, found the track it should be on…
So my beginnings need more editing because I don’t yet know what’s going on, but I figure it out. Taking a pause when you’re stuck to think through where you’re going has been helpful to me, so the characters don’t just run around in circles as you build words. I never really saw the point in writing something I didn’t at least think I might want to read later.
I’ve also been setting goals for myself over and above the 50,000 words - once I knew I can reach that, that is. My first year, my only goals were 50,000 words and a finished story. (I.e., no “I’m at 75,000 words, it’s the end of November, and my ending is nowhere in sight!!!” - something seen on the NaNo forums far too often.) My second win, I was playing with a different story format, doing fixed-length episodes. Time to figure out my next goal. 50,000 words, a finished story, and …
Oh, and make sure your family understands and supports what you’re doing. The only reason I managed last year is because my husband agreed to do the cooking and cleaning, a job normally shared - I could get home from work and get an hour of writing in before dinner, instead of spending most of that hour making dinner.