"There's a moment just before every rep when you are faced with a choice: You can either focus your attention on the target (what you want to do) or you can focus on the possible mistake (what you want to avoid). This tip is simple: Always focus on the positive move, not the negative one. . . .
A violinist faced with a difficult passage should tell himself, 'Nail that A-flat,' not 'Oh boy, I hope I don't miss that A-flat.' Psychologists call this 'positive framing,' and provide plentiful theories of how framing affects our subconscious mind."
So, do we ask our choirs to focus on the positive, on the goal? Or do we say, "Watch that pitch, it's a little under?" Demonstrate (play or sing) the correct pitch and make that the focus.
When the ensemble isn't precise, have them count-sing and focus on singing precisely together, then (when the count-singing is together) have every other one count-sing and and the other half sing the text, making sure that the consonants are line up exactly with the count-singing members of the ensemble. Then switch it around.
What are other ways you can make sure your ensemble focuses on the goal, not avoiding a mistake?