How do you plot?

Plotting for me has become a necessary thing. It didn't used to be and I still start stories on a whim, but I do plot out the arc of the story now. I find it very valuable and it keeps me from writing myself into a corner...most of the time... The characters still manage to go off on tangents and stump me of and on, but for the most part, I at least have a map to look at to direct their journey in the right direction. Doesn't mean we won't go southwest, when we were supposed to go south, but...