I think that monkey see, monkey do. The learn from us. Also, in saying that, it is good to get into the habit of being consistant with your reminders of please and thankyou and you will be rewarded.
My sister is very inconsistant with her boys and rarely asks them to say thankyou or please. They are out of control and a complete embaressment when we even attempt to go out with them.
You are doing a great job, manners are so important when raising children..