Let's start by defining what manners really are.
They are the social actions we take in order to be easily accepted by others around us. The ultimate goal, in my opinion, is simply to gain cooperation and camaraderie.
That sounds like a pretty good deal!
There are those who disclaim the niceties of saying "please" and "thank you" and yet when these so-called mechanical and meaningless trivialities are employed, human relations seem to flow.
I have to admit, I unashamedly use this "weakness" of human spirit to my advantage in heavy traffic situations where I put on my blinker and wave my hand in a friendly, "might I get into your lane?" gesture. First off, I have NEVER had anyone refuse me the lane change. Sometimes the obsessive texters don't see me and drive on oblivious to anything around them as usual. But, do you know that the folks who are actually paying attention seem completely pleased to be able to "help" a stranger and we both drive off with smiles on our faces.
You can consult your own reality on this by recalling a time when someone politely asked you to wait for something - as in a line at the bank - and then thanked you for being patient vs. a time when you were bluntly told to wait and then handled impatiently by the service personnel once your turn actually came around. Same wait time. Utterly different experience and opinion of the institution.
If you think your child will have a better experience in life if he or she gets into the habit of being pleasant to those around him, start now with having good manners within your own family.
Honestly, stop yelling at your kids and your spouse. Rather than barking orders at little Johnny, try asking him nicely if he wouldn't mind taking his dish to the sink before going out to play. Let him know that it would really help you and make a big smile happen on your face! (or something rather playful… not heavy and serious). If he refuses you can lightly thank him anyway for listening and do a little dance over to the sink with the dish and say "Mr. fork is still there on the table…. If you'd like to bring him over to me, I think a big hug might happen!"
You get the idea.
The funny thing is, the more thoughtfully you treat others, the more thoughtful they seem to become. It is not an easy road and your patience will be constantly tested but your smile to frown ratio will change markedly - and in the right direction.