Preparation is obviously very important in teaching. I've occasionally done a bit of teaching off the top of my head but I've found I'm pretty much always going to forget something if I don't prepare.
I teach a bunch of numeracy classes (6 in total) and I find them quite rewarding but it is very difficult to remember exactly where each pupil is week after week. Almost each class has pupils learning at each of the different levels and its hard to keep track of exactly where each pupil in each class is even with a written record. I need to double check exactly where each pupil is the day before the lesson, so I have the correct materials for each and every one of them.
That's not to mention the planning needed to make sure you have everything when you don't teach all of your classes at your normal building. For instance I forgot to bring protractors from my office to my teaching today and had to rearrange my teaching plan on the fly.
I'll hopefully get better at remembering to write things down.