If I'm doing something local, I usually go with at least a vague plan of how I want to shoot the "primary target". Often, I'll estimate the best time of day, etc. of a previous scouting. Once there, however, I also use the framing aid to determine camera position, composition, and preferred focal length.
If I'm taking a trip to an area I'm not already familiar with, I'll do research on the Web, and look at topo maps. The topo maps, along with a solar position calculator, helps in figuring out in advance where to be at what time of the day. For commercial-style shoots, I'll often do sketches of how I want to compose the object, what things I might want in the background to help tell the story, and how I want to light it. For more complex things that might involve a series of images, I sometimes even do story boards, where I plan both individual shots and the sequence.
Actual compositions are usually determined by deciding what needs to be in the scene to "tell the story", how I want to balance things within the composition, and what I need to do to focus attention where I want it. I try to be aware of what is going on, too, so I'll sometimes wait for a cloud to move into place, a bird to fly through, a wave to break, or whatever. In this shot, for example, I waited about 20 minutes for the spot of sunshine to move onto the trees: