"You can't be truly rude until you understand good manners."
-- Rita Mae Brown
Today's manner is a simple one. Often children are given compliments or treats by others. Just as often, they forget to thank the person. Most parent's first instinct is to say something like, "What do you say to so-and-so?" with that expectant pause at the end where the child fills in the "thank you" blank. This is not the proper etiquette. The best way to teach manners to children is to model them. Instead of prompting the child, which if you think about it is very condescending, you should just say "thank you" to the person.
Since I've stopped asking my kids what they've forgotten, and just filled in the blank myself, I've noticed that my kids forget less and/or thank the person right after I do. I don't know about your kids, but mine are excellent mimics. They love to say and do the things that I do. It's not really that surprising that they should also copy politeness. Actually, this goes for just about anything where you want a child to say something. If they accidentally bump into someone, apologize for them. If someone sneezes, go ahead and wish them good health. Mine often forget to say please when asking for something. I sometimes repeat the request with a please of my own. I don't try to correct their statement, instead making one of my own, "Could Dimples have a cookie please?"
Later, when I'm alone with the child, I talk to them about being polite. That way the kids feel better about not being corrected in public, and you can still express the importance of manners.