Dumbbell shrugs, but not typical standing shrugs. Lean forward slightly, and use your off arm to brace yourself against a bench or rack or something that's at about lower chest level. You want to be about 30 degrees down from vertical. Do single arm shrugs with medium weight. Many guys use too much weight for shrugs and it limits their range of motion. Focus on feeling the contraction at the top of your trap, toward the back of your neck. Hold the contraction for 3-5 seconds at the top, and go slow on the negative. Keep your arm straight and don't bend at the elbow. Remember that this is a shrug, not a row.
Traditional shrugs don't work the rear most part of the trap as well, and they limit the thickness of the trap that ultimately helps to make the muscle stand out and look larger.