1. The less I teach, the more my students learn. I'm slowly learning not to worry about "getting through" all of the material I've planned to do, to embrace the occasional tangent, and to not cover everything in lecture--to let the students learn some stuff on their own. Strangely, covering less material seems to make my students better learners. (I now realize why, but it took me a while to learn and embrace this concept.)
2. My students tend to be very uncomfortable with "unconventional" teaching styles--which to them is anything beyond lecture. But if I'm patient, and if I construct good classroom activities, even computer science majors will eventually embrace them--even the "dreaded" discussion format.
3. If I am being observed, I will do at least one boneheaded thing during the class, such as claim that 20/2 = 18.
4. I should always, always have a backup plan. Particularly if my primary plan depends on the students having done the reading. (Corollary: Making up a lecture on the fly is hard.)
5. I really need to get in the habit of writing all of my assignments before the start of a course (as opposed to determining the topic of each assignment before the course starts, but deferring the actual writing of the assignment to the day before I assign it). I think that doing so will make my classes flow much better.