My method is really similar to Tad's.
Scribbled notes over the course of a week, read through books for ideas. Reviewing the previous game and trying to figure out the details that I handwaved during the last session is a big part. Rough outlines for where I think the players might take things are super helpful.
For overall campaign/scenario planning, I tend to do a sort of flow chart of important events, objects or people, and ways that they might lead to one another. I put the items in boxes, and the checks or events that might reveal a clue along the arrows connecting them. It helps me to rough out the entire "web" of the story without making it linear, and in a format that doesn't take long to reference or build.
One thing I really like is tools to help me track my improvisation during game. I use a "badguy tracker" with a bunch of blank lines for stats/health of any enemies I might come up with on the fly. I bring a sheet of genre-appropriate random names in with me, and space to make notes next to each one.
In general,, I'd say I put in about 1 hour per hour of gametime, but it is spaced out in 15 minute intervals over the course of the week, with maybe an hour of dedicated time right before the game starts.
I also have someone like Setherick Patrick to help me bounce ideas around. I actually have a few. Violet (my girlfriend and an avid gamer) is a great sounding board for concepts, but I can't usually bounce plot off her because she's playing in the games. My friend Hero is a great GM but rarely has time to play, so I tend to run plots by him. Ross has been helping me out a bunch lately, because he's awesome.
