When asking for something, say "Please."
When receiving something, say "Thank you."
Do not interrupt grown-ups who are speaking with each other unless there is an emergency. They will notice you and respond when they are finished talking.
If you do need to get somebody's attention right away, the phrase "excuse me" is the most polite way for you to enter the conversation.
When you have any doubt about doing something, ask permission first. It can save you from many hours of grief later.
The world is not interested in what you dislike. Keep negative opinions to yourself, or between you and your friends, and out of earshot of adults.
Do not comment on other people's physical characteristics unless, of course, it's to compliment them, which is always welcome. (Read more.)