My traps grew best initially with heavy-ass barbell shrugs. However, lifting big weight eventually leads to diminishing results. With a lower back that can't handle spinal compression, I resolved to set meathead tendencies aside to learn the most effective path to trap growth:
This is accomplished in a couple different ways. The first and simplest is to hold the weight in the top, contracted position for two seconds on every rep. This increases the time under tension.
The second is to add bands to whatever you're using to hoist weight – barbell, trap bar, or Dead-Squat® Bar. The bands are particularly good because it causes you to fight for the peak contraction vs. just holding it. You're continuously working against gravity and a band.
Lower the weight with a controlled tempo and immediately reverse direction at the bottom without resting.
The Dead-Squat® Bar is excellent in this regard. I feel the contraction better when I'm able to lift and lower the weight from a neutral position rather than with a barbell out in front.