All aformentioned advice is good, although I'm not too sure about the "rolling the shoulders forward" quote, but whatever. The key for shrugs, imo, is not to go too heavy. You should not be using any momentum that might be aquired by pushing off with your legs, and thus causing inertia to be about 50% of the reason the rep was completed, as opposed to 100% of it being muscle if the weight is lifted correctly. Lift the dbells up with good form. No jerking. No throwing your back into it. Pause at the top briefly, and extend back down as far as possible. Repeat.
If you want heavy work for Traps, then Cleans(as previously mentioned), and Deads are where your focus should be. NOT on shrugs. Just my opinion, as I know there are dudes here who are heavy shrug freaks. I'm sure if they say they work for them, then they surely do. I'm just saying that there is no need to go heavy on Shrugs. Again in my opinion, it isn't necessary, you cheat yourself if bad form is used(and it most often is when going heavy on shrugs), and most importantly, you are at risk for BIGTIME INJURY performing heavy shrugs. Especially with a straightbar. You don't want a cervical strain, trust me!