Maintain good posture. Standing straight also sends the message that you are confident. Get in the habit of holding your head comfortably high, but keep it in a relaxed position. When you hold your head comfortably high, your body will naturally have good posture. Try this posture exercise: pretend you have a string attached to the top of your head. Imagine the string is being pulled straight up until you are standing tall and holding your head in a comfortable position. When you find yourself slumping, do the string exercise.
-Renee Evenson, Customer Service Training 101