The trapezius has a "trapezoid-like" shape, hence its name.
The trapezius is a long muscle that goes from the back of your neck down to your middle back. It is comprised of three regions known as the upper, middle and lower trapezius. Your trapezius is important for good posture, as it plays a large role in keeping your neck and upper back in proper alignment. To best train your trapezius, you must do an exercise for each its three regions.
The best exercise for your upper trapezius is the shrug, which is a straightforward shrugging of your shoulders. Hold a dumbbell in each hand, stand up straight with the dumbbells by your sides and then elevate your shoulders in a shrugging manner. Simple enough, but highly effective for building stronger upper trapezius muscles. Try to visualize your shoulders almost touching your ears with each repetition to get the best possible contraction at the peak of the motion. Hold this contraction for up to two seconds per repetition.
This region of the trapezius is responsible for retracting your shoulders, such as when you squeeze your shoulder blades together. That is the exact exercise you want to perform for this muscle, and it's called the "T" exercise. Grasp a dumbbell in each hand with a thumbs-up grip. Bend forward so your back is straight and parallel to the ground, holding the dumbbells below your torso with your arms straight. Squeeze your shoulder blades together, thus lifting the dumbbells upward, and hold the contraction for two seconds. Then relax your shoulder blades and repeat.
The lower-most region of your trapezius also retracts your shoulders, but its main action is to depress them. Shoulder depression is the opposite movement of shoulder elevation, so you have to do the opposite exercise of the shrug. One such exercise is the dumbbell "Y." Lie face-up on an incline bench, holding a dumbbell in each hand with a thumbs-up grip. Start with the dumbbells close together and your arms straight. Lift the dumbbells up and away from your body in a "Y" motion. Hold the contraction at the top for two seconds and then lower the dumbbells.
Doing merely one set per exercise will not do much to build your trapezius. An effective volume count is four sets per exercise, for a total of 12 sets of overall trapezius training. It is best you do the exercises in tri-set fashion, meaning you do one set of dumbbell shrugs, followed by one set of the dumbbell "T" and then one set of the dumbbell "Y." After the third consecutive set, you take a two to three minute rest and repeat. Perform 12 reps per set using medium to heavy resistance.