Don't ask the guy with big traps what exercises he did to build them. In all honesty, there's a 50/50 chance he was just born with awesome trap-building genes. Instead, learn how the muscles work and learn what it takes to transform your non-mutant traps into great traps.
Testosterone, growth hormone, mechano growth factor, and insulin-like growth factor are all massively influential on growth-signaling cascades, and you don't need to commit to a shot in the butt just to maximize the benefits of these substances. They exist in your body as it is, and resistance training effectively releases them (1). However, it's leg day that seems to really release the floodgates (2).
So what does this have to do with your traps? Well, your traps have some of the densest amounts of androgen receptors in your entire body. These receptors are responsible for the uptake and utilization of testosterone (among other hormones).
So let's say you've just survived a massive leg workout. You've got those hormones mobilized. If you take the opportunity to train your traps right after legs, you're going to increase the potential for growth. In short, train traps after legs.
Bouncing, full-body shrugs using 1000 pounds doesn't work. Every guy with mediocre traps in your gym does that, right? Instead, use a lighter, but still challenging weight. Think "long stretch" at the bottom of the movement and "paused, high squeeze" at the top. Do this for every rep.
Remember, the trapezius is a big muscle. There's a lot more to it than just the part that crests out of your tank top. You'll have to work it from different angles. Try these exercises:
Full-Range Seated Barbell Shrug
Use a reverse grip and small circumference weights for a larger range of motion.
Explode up, pause at the top, and lower slowly.
One thing powerlifters, CrossFitters, and Olympic weightlifters all have in common (at least the talented ones) is that they all have massive traps! Why? Because the very act of holding heavy shit in your hands stimulates trap growth. Make cleans, farmer's walks, deadlifts and related exercises part of your training.
- Kraemer WJ et al. Acute hormonal responses to heavy resistance exercise in younger and older men. Eur J Appl Physiol Occup Physiol. 1998 Feb;77(3):206-11. PubMed.
- Ahtiainen JP et al. Heavy resistance exercise training and skeletal muscle androgen receptor expression in younger and older men. Steroids. 2011 Jan;76(1-2):183-92. PubMed.