Some people like to work on the same project the whole month, some people have a few projects, others count anything they've written towards their word-count goal. I know a bunch of participants start writing early to meet their goals during the month. There aren't very many hard rules to NaNo other than "don't steal someone else's work". Apart from that you can basically do whatever you want.

EDIT: Fishy is right in that's what you traditionally do, but I've seen a bunch of people either do something completely different during NaNo (like editing, which I did one year) and I've seen people start writing early to help them deal with time constraints they knew might keep them from getting to their goals in a month.