How To Perform The Seated Shrug
Grab two dumbbells and sit at the edge of either a bench or box. Bring your legs relatively close together, so they are out of the way of the dumbbells.
Use the cue, "superman chest." Imagine you have the Superman logo on your chest and show it off. This will open your chest and force you to sit tall while pulling your shoulder blades back.
Let your arms be in a dead hang position.
Performing the Seated Shrug
Pull your shoulder joint straight up toward the ceiling. Get your shoulders as high as you can. You should feel your traps contract very hard when you have hit an adequate range of motion. From this point, begin to drop your shoulders back down to the starting position.
Avoid rolling your shoulders. This is a very common form mistake on shrugs. The upper portion of the traps elevates the scapula, while the mid and lower traps, retract and depress the scapula. However, when you are in a seated or standing position the dumbbells will be pulling downward (because of gravity) so scapular elevation is the only anatomical function being performed against resistance. Therefore, retracting and depressing the scapula on each rep (rolling the shoulders) will not recruit your mid and lower traps. You would need a different exercise for that.
Also, pay particular attention to your neck. It is also very common to push one's head forward when performing a shrug. This can put unnecessary stress on your neck and cervical spine. Keep your chin pulled back and your head in a neutral position.