I have a 13-year old and a 10-year old who are constantly being complimented on their good manners. Don't forget: parents should be modeling these behaviors themselves!
1) Insist on "please" and "thank you" from the beginning.
2) Teach them to show an interest in, and ask questions of, other people.
3) Have them write thank-you notes. NOT thank-you e-mails! Actual notes. Buy them fun personalized cards to use.
Most all good manners can be boiled down to some form of these three tips.
Parents have to be role models. They should always praise their children later on when they noticed their good manners and talk about how much you admire some people for their manners.
I frequently comment to my friends children, " I love the way you say please.", "Thank you for being respectful." "I appreciate that you let someone go ahead of you on line.", "Thanks for waiting patiently." Its amazing how they always respond with their best behavior around me.