"My Traps Suck! What Should I Do?"
Here's the obligatory "strong guy" answer: if you want big traps you should deadlift regularly. But in reality, the traps are only contracting isometrically (without movement) during a deadlift, but they're under an intense load while in a stretched position.
You CAN use targeted exercises to make the traps grow, if you do them properly. There are two issues at hand: how you execute each rep and what movements you do.
The first problem people have when they do direct trap exercises like shrugs is similar to the problem they have when training calves: the range of motion is very short.
If you do your reps too fast – which happens when you go too heavy – the muscles won't be under tension for a long enough time to stimulate growth. And it's even worse when people cheat on the way up with their legs in order to shrug more weight.
The Solution for Sucky Traps
An effective isolation rep for the traps is fairly slow on the way up, includes a 2-3 second squeeze at the top, and is lowered slowly until the traps are stretched.
And of course, no cheating.
When doing shrugs, try bending forward just a few degrees. This gives the middle fibers of the traps more play, which will give you more overall thickness. It also seems to put less strain on the levator scapulae, which often gets inflamed when doing shrugs.
People normally train only one function of the traps: the elevation of the scapula (shrug). But there's another important function: the upward rotation of the scapula.
Watch this video. The third exercise showcases an upward rotation of the scapula:
While my goal isn't necessarily to have you do that superset, it illustrates both important function of the traps.
My favorite way to train the upward rotation function of the traps doesn't even require that you add another exercise to your program. When you do overhead presses simply try to shrug at the same time as you press the weight. This works especially well with dumbbell presses for the shoulders.
As soon as the elbows break the shoulder, start to shrug as you continue to press. This is one of the most powerful ways to stimulate growth in the traps.