I don't _really_ plan... Just improvise until it all comes together in a glorious masterpiece, that will span the ages... Of 20 to 40.
I plan, but with D&D-like stuff, I like to leave openings and holes, so that I can improvise at what I think is needed. I think my best DM ability is improvisation. Having read and played so much, that I can improvise almost whatever the players want... But that's just a guess. It's hard to accurately, and fairly, judge one's self.