While 12 reps and or holding the contraction may work very well for trap mass, it is also imperative that you train them for pure strentgh instead of size, cause traps play a mayor role in pressing movements if you want build upper pec en delts!!!
I actually do the trap strength training the day before i do mass training.
DAY 1: trap strength
shrug machine: 6-4-4 reps
DAY 2: trap mass (after training delts)
shrug machine: 8-6-6 reps (hold contraction 2 seconds)
dumbell shrugs 10-8-8 reps (hold contraction 2 seconds)
close grip upright row: 10-8-8 reps
medium grip pulldown to neck: 15-15 reps
I like to finish training a muscle by doing 2 sets of 15 reps for a final burn/pump