I'd like everyone to post one saying, one saying only, any subject, that they would answer in response to the question "what is the most useful thing you can tell me?"
I'll start it off, not by saying mine, not yet anyway, but by repeating the words Peter Brideson gave me when I asked him this question many years ago.
"You can say anything you like to your boss, as long as you are not right, that is the one crime they will not forgive."