This year, I'm actually managing to built the buffer I always need toward the end of the month. I know, going into NaNo, that I need to buffer at least two or three days (for my daughter's birthday), but in the three years I've been doing this, I never did it. This year, I not only managed to keep ahead of my daily goals, I also had no trouble catching up on two days I lost to a severe cut in my finger (five stitches!).
Now, how did I do that (edited to add: I meant writing so much despite the finger, not cutting it ;-) )? I used a tool by David Gale (still called NaNo-tracker, the name will change, soon) that automatically recalculated the amount of words I had to write the next days when I entered how much I did write that day. It's a really, really hand tool, not only for NaNo. He promised to adjust it so it's flexible enough for longer time periods and other months. I guess with it, I'll be able to double my productivity.
Good luck to all NaNoists, no matter how many words you managed. It's always a valiant try.