Some things I've learned to plan (more on that), but most of the time I will have a general idea of what I'm going out to shoot (sunrise/sunset, frogs in the water, general wildlife, clouds, etc.).

One evening, the Marine Corp band was going to be playing on the steps of the Capitol Building in DC - my father (also a photographer) and I went out with our medium and large format cameras to "get the band".

I plopped out my tripod, quickly mounted the camera - may dad was following suit - and began taking meter readings when a cop walks up to us and asks, "Excuse me, but do either of you two have a tripod permit?"

I look over to my right and see a guy walk right up beside us with a point and shoot and snap a picture - NO TRIPOD! 45 minutes later - the band already gone! - we had a 1-week free permit to shoot on ONE QUARTER OF THE  LAWN. The full permit requires 4 signatures and the other 3 guys had left for the day, leaving just this one guy to sign on on the front right quadrant... >:-[ We managed to get some good shots of the building at night, though.

Some things you HAVE to plan, but most times I just go out and shoot.
-Lew ;-)