In the times when I've had to be in front of a class, I've had to be able to ditch any kind of "lesson plan" which I had in mind to make sure that what I was doing up there reflected the natural course of the class. Many teachers I have met (in fact, I'd say the majority of them) have admitted to the same kind of flexibility being necessary as well.
I remember talking to Frank Doran sensei this summer about teaching. He said that he treats his classes like a laboratory. Sometimes, what he teaches doesn't "work" and he lets it go. Other times, he notices that it's working pretty well so he puts it into his mind as something he can try using some other time.
I don't know how many ways my current teacher has explained such concepts like kuzushi, musubi, and timing to me -- from pushing a car to treating uke as a frying pan to balancing a jo on one's hand, he's always thinking of different ways of letting people understand such concepts.
I think it's an important aspect of teaching in any kind of venue (and not just in aikido) to make sure that the teaching isn't just a one-way conduit. The teacher has to keep learning what's working and what's not working. As a friend of mine related, George Simcox sensei said that he would stop teaching if he himself failed to learn something new for three nights in a row; he never quit...
As an aside from a student's perspective, I enjoy teachers who are teaching things that they're currently working on rather than, say, a teacher who comes out and shows some sort of "finished product" (if there's such a thing). Show me a "work in progress" any day.