Sometimes being a parent can be like being a broken record: "Don't do that!" "Be careful!" "I've told you three times to clean up your room!"
But sometimes catching our children being good can be much more effective. We must strive to notice any improvements or we'll be like the mother described by Alan Loy McGinnis in his book Bringing Out the Best in People:

"A little girl had been misbehaving and her mother had to rebuke her often. But one day the little girl had tried especially hard and hadn't done a single thing that called for a reprimand. That night, after the mother had tucked her in bed and started down the stairs, she heard her daughter sobbing. Turning back, she found her head buried in the pillow. Between sobs her daughter asked, 'Haven't I been a pretty good girl today?'

"'That question,' said the mother, 'went through me like a knife. I had been quick enough to correct her when she did wrong, but when she had tried to behave, I had not noticed it. I had put her to bed without one word of appreciation'" (1985, pages 113-114).