"Imagine not being considered for a job as a result of failing to sit or stand up straight," says imageconsultant Judith Rasband. "It happens. Poor posture .... can hinder the message that body image presents."
Indeed, she argues, posture can send a message you don't want sent. If you stand up too straight, you may "exude an air of arrogance," while if you slouch, you convey "fatigue or low self-esteem." More practically, poor posture at work can lead to ergonomically related conditions that reduce productivity.
So Mom was right about standing and sitting up straight. But sometimes we forget. Rasband says that if you stand with your heels about an inch from a wall, the back of your head, shoulder blades, and buttocks — and nothing else — should touch the wall, and you should be able to fit a hand behind the small of your back.