Although the upper traps are already worked a lot during the typical training session, the traps still need focused retraction and depression work to prevent imbalances. This exercise targets the rhomboids and middle/lower traps exceptionally well:
The Kelso Shrug
Lie face down on an incline bench with your chest supported and feet firmly planted on the ground. Be sure to create a stable base with your feet.
With an underhand, shoulder-width grip, grab the bar and lift it by pinching the shoulder blades down and back as hard as possible. Focus on getting a hard contraction of the lower traps. Try to "break the bar" by externally rotating the arms. This makes the shoulder blades come together nicely.
The Kelso Shrug With Dumbbells
Extend the upper back and focus on contracting the lower traps through external rotation of the hands/arms.