One of my favorite authors is Elizabeth Bear, who maintains a slice of the writer's life blog that I find fascinating. She likes to say that "writing is like everything else" because of the number of analogies she and other authors come up with to explain the writing process.
Teaching, I suspect, is not quite like everything else, but planning is like writing and other acts of creation. You know: sometimes you get a great idea in a flash of intuition and you go into flow and it just keeps coming, sometimes it's like pulling teeth to get those words on the page but you have to do it, and sometimes you just plod along but the words get there somehow. When all's said and done, you wonder if the readers can even tell the difference between intuition like fire in your brain and the long slow pull.
The reason this connection between planning and writing is interesting to me is that it explains why I get so much more done on my last period planning days than my 2nd period planning days: flow. When I know that if I get into a groove I can just keep working until its exhausted, I'm much more likely to make the effort to put myself into the focused state of mind I need to plan. When I know I'll have to go back to class (not teaching--running a study hall, ugh) at 9:55 no matter what I'm in the middle of, I'd rather do something mindless like enter grades and copy things.
All that considered, no wonder people complain about students not thinking and not engaging with the lesson: why should they put the effort in when no matter where they are, the bell will ring and they'll have to go to their next class?