Encourage interruptions; don't be afraid to digress:
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This is a consequence of the one just above.  The point is that the reason
for the course is for the student to learn.  If the student is not learning,
it it irrelevant how much material is being covered.  Therefore encourage
interruptions.  It means the student is thinking.  And, if the student is
thinking, he/she has a chance of getting it.  Digress when it seems useful.
Digressions can add meaning to the understanding of a concept.  Anecdotes 
from the instructor's experience can make the material more alive.  It can
provide just the twist that allows the student to get it.