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

Whether or not you were a dancer, you've almost certainly had someone in your life tell you that need to sit or stand up straight.  While they were probably trying to make you look less like a slob or to avoid you having back issues as you aged, little did they know they were actually giving you excellent career advice.  This little action of standing or sitting up straight is not to be underestimated.  As Renee Evenson points out, we naturally feel more confident when we have good posture (and others tend to perceive us this way, too).  We also tend to sound more confident when we speak when we have good posture, so sitting up straight has benefits even when our clients and colleagues can't see us.  So, at least in this case, your mother was right (thanks, Mom).