Dumbell shrugs, Barbell shrugs to the front and back built the bulk of mine. And deadlifts are doing pretty good for them now as well. Though traps was always my best growing body part and the first to be noticeable on my skinny frame. Sometimes Id hit 12 sets for traps alone, but often I did 6-8 sets a week as well. Between 12-15 reps per set.
besides deads and rows, do various shrug exercises...
wide grip barbell shrugs - sets of 10, increase weight until you can no longer do a set of 10 (peak builders)
close grip barbell shrugs - 2 sets of 10 at a comfortable weight, not to heavy or to light
db shrugs - help widen your traps