When I first started to teach yoga I planned my lessons in the finest detail -- what we would do, how we would do it and exactly how long it would take.
Gradually over the years my lesson plans have become less and less detailed. I have learned that there are times when certain postures/sequences are out of the question depending on who is in the class and what the energy feels like. I have learned to gauge what my students want to do rather than what I want to teach and so I lean more and more towards the stance of making lessons up on the spot.
Obviously left completely to my own devices things could get chaotic so I always have a theme and a central posture that we will work up to and down from. Vinyasa krama - step by step.
For example, last week my theme was legs and hips, my central posture was parsvakonasana - working on the energy from heel to fingertip. I teach two general level classes a week (my other classes are specialist; therapy or pregnancy), and the way we worked up to and down from these postures was quite different in each class. The energy was high on Wednesday and I could tell that the class needed a strong practice. On Thursday, however, the energy was quite different and I needed to work on a more restorative flow into the central posture with longer in savasana at the end.
It used to drive me wild when the randomness of life meant I couldn't stick to my lessons plans, but over time I am beginning to cultivate the notion of flux. Just allowing myself to roll with whatever my students need. And with this knowledge I grow to love teaching more and more and more each day!