It is not necessary to write down EVERY word I say. Look for the 3 S's:
If I slow down when I say it.
(This is a big clue that I am giving you time to write.)
If I stress it over and over.
(If I am repeating something, it's probably important. If you have already written it, then underline it, then you'll remember it's important when you review it.)
If I stop to write it on the board.
(Especially if it's a diagram or chart.)
Clark, Ron: The Excellent 11, 2004.