Before you are picked up, gather all of the items you are going to take with you.  This way when your parents arrive, they won’t have to wait while you run around the house looking for your things. 

As you leave, make sure you have cleaned up after yourself.  Anything you used or played with is put away unless the host or hostess asks you to leave it where it is.  Any food or drink is properly disposed of.  And any general mess is taken care of.

Finally, as you leave a party, dinner, or any sort of get-together.  Let the host or hostess know you appreciate all of the effort they put into hosting the event.  When people plan a get-together, they generally put time, thought, and effort into deciding the menu and activities.  Let them know their efforts are appreciated.