"Treat everyone with politeness, even those who are rude to you - not because they are nice, but because you are."
B suggested today's manner:
"Manners are not about people getting what they deserve."
Regardless of how you have been treated in the past by a person, your manners toward them should never change. This is a hard one for kids. They seem to be acutely aware of any sort of injustice in their worlds. My mom always used to say, "Two wrongs do not make a right." That phrase drove me crazy as I kid, but I've found it in my head frequently as an adult. Now I say it to my kids, although I try to rephrase it and explain why we don't just retaliate. It's really just another way of phrasing the "golden rule", isn't it?