"Plan less, prepare more" is definitely the way to go.
It depends on the specific group, but it's so fucking frustrating when you spend hours planning this grand storyline, and players start completely screwing it up within 15 minutes of the session starting. Creating a general plot-framework for players to muck around in instead of a specific plot is a far more worthwhile investment of time.
I like to supplement the plot-framework with world-building, so I always have extra material to throw out there when things take a severe left-turn, and my players get to do whatever they want while I gently nudge them in the direction I want to go.