My best work comes from planning. I've been at some locations after visiting it many times, studying the light and the sky (and sometimes consult an ephemeris and topo map to know just where the sun will some up) and then I'm in the right spot at the right time. And them some fellow walks up, surveys the scene and what I'm doing and says something like, "you sure are lucky to catch it on a day like this!" I never respond to a statement like that, but you can guess what's going through my mind.
Thing is, I do get the occasional lucky shot using, as Jim mentions, a wary eye for anything interesting. While I like these sorts of shots a lot (much payback for little effort) they are rarely my best work. After four years of serious shooting I have only one shot that came out better on the first vsit than on any subsequent visit (you can see it here: http://chem.dynu.com/photo/images/1026.jpg ).