My girlfriend, who's an excellent teacher in her own right, has offered another suggestion about teaching that I heartily endorse:
Sarcasm has no place in the classroom.
The few times I have used sarcasm in the classroom, I have always regretted it. It tends to alienate students, even those who aren't the object of the sarcasm. (I'm reminded of the Pink Floyd lyrics: "No dark sarcasm / In the classroom.")
As a corollary to this point, I'll say that sarcasm has no place in lectures about history. What I mean is, the historian, or teacher of history, should usually strive to present the topic without being sarcastic about the intentions of the historical actors involved.
For example, if I'm teaching about the so-called "Lochner-era" of the Supreme Court, the mantras of "the Court gave people the right to work extra long hours without overtime" (Lochner v. New York) or "the Court affirmed the right of people to be cheated in a lottery scheme" (I forget the case, but it came from the 1890s and had to do with a Louisiana based lottery) do not necessarily capture what the justices meant when they cast decisions based on "liberty of contract" doctrines. (For what it's worth, I realize that the notion of a "Lochner Era" court is a bit under dispute and a bit of a caricature; still, the use of sarcasm does nothing, in my mind, to expand critical thinking about the issue. Also, David Bernstein at the Volokh Conspiracy is working on a book about the Lochner court, which I look forward to reading when it's out.)