Technique Summary

Holding barbell with a slightly wider than shoulder grip, raise your shoulders to your ears.  Exhale on the way up.  Inhale on the way down.

Technique Details

Stand comfortably with feet at shoulder width.  Take an overhand grip on the barbell.  If you are working out with heavy weight you should take an overhand grip with one hand and an underhand grip with the other.  This will prevent the bar from rolling.  To keep the strain on your traps even, change the grip with every set (i.e. first set, left hand under, right hand over,  second set, left hand over, right hand under).  Keep your back straight and your chest out.  Keep your legs straight without locking out your knees.  Your body should be frozen in this position as you perform the exercise, with only your shoulders moving up and down.  Controlling the weight, bring your shoulders as far up as you can towards your ears.  The trapezius is a muscle that goes from behind your neck down the middle of your back.  To exercise it your shoulders should always be brought up to your back, not the front (while on the topic, this exercise only targets the upper portion of the traps).

Don't try to rotate your shoulders when doing the barbell shrugs.  Don't flex your arms, bending at the elbow.  This shifts the focus from your traps to your bi's.  Don't push yourself up from your legs.  This shifts the focus to your legs.  Keep you head facing forward.  Don't move it around.  Traps are especially prone to injury.  If you move your head around and jerk the weight, you could find yourself in the position where you are stretching and flexing the muscle at the same time.  This leads to overstressing and muscle tears - not deadly, but definitely painful.