I think what is missing from most parents is the follow through
I hear constant streams of "if you don't do XXXX then I'm going to YYYY" from parents but they never actually seem to do it. If they actually do try to follow up the kid will scream for 0.0005 seconds and the parent instantly gets all apologetic.
Don't be afraid to let kids scream a couple of times. Totally ignore them when they do it. They'll soon figure out it doesn't work.
Pet hate #2 is parents who constantly ask kids what they want to do today, what they want to eat, or whatever. You don't ask kids what they want, you tell them. You put whatever it is everybody else is eating on a plate in front of them and tell them to eat it. If they don't eat it, they go hungry.
Be the boss.