My own two-cents about NaNo? I've completed it for the last two years in a row - the first year, I went in after deciding on the second-last day of October that this would be fun, and came out with a manuscript I love. The second year I spent months preparing, and came away with a manuscript that fell painfully short of what I had hoped to achieve. Both years I finished, but I guess the best advice I can offer is to not have expectations for yourself. If you want to plot some things out, fine, but don't discount yourself the ability for spontaneous plot twists and character development, because that's what made the NaNo experience so worthwhile for me!