As a young manager I used to be stern, demanding and forceful because I thought that that conveyed authority. I still tend to say things twice in a row with different words when I want to get a point across to my children. But I am beginning to learn that people get it even when I don't hit them over the head with a hammer. As a matter of fact, we (and animals, too, by the way) get it even better when we formulate a request in the affirmative. How would you prefer to be corrected? "Stop yelling" or "I can hear you well?" I ask my daughter to use her "morning voice" when she speaks loudly at the breakfast table. She gets it.
I read that the universe doesn't understand the "not" part. We are similar, as are animals. We understand better what another person wants (and it sounds much nicer, too) if she says in kind words what she wants, instead of reprimanding what she is critical of. Instead of "don't be late" why not try "please be on time, we begin at 8AM sharp?" Instead of "your table manners are terrible" try "fork in the left and knife in the right hand." Instead of "you forgot your homework again (grumble grumble)" try "it makes my life a lot easier if you hand your homework in on time, and I can give you a better grade, too." Instead of "don't you scratch my couch" try "here is a great scratching post for you."
Drop the hammer, pick up a feather.