Different campaigns, different styles. In my Planescape campaign, I plan in meticulous detail. I don't write "notes" beforehand, I write commercial-level adventures. I also make multiple plans for the most likely player decisions. Finally, I also write down setting details such as NPC motivations and resources, timelines, events that happen off-screen, etcetera. These sort of things allow me to keep the adventure consistant and under control even when the PCs do something completely unexpected, without railroading. The adventure I'm currently running is longer than my thesis and still not close to finish. If you don't believe it, check my sig.
What can I say, I'm probably insane... but that insanity doesn't extend to my Warcraft campaign. There, I only write a couple sentences about what the overall plot is going to be, and I flip through the MM before the session to pick up a few things to fight. I adlib the rest.
The two campaigns, needless to say, are very different in style. They are both great fun, but in different ways.