Power Cleans and/or Hi-Pulls off a power rack. Both affect the posterior delts also. Try a trap bar or even DB's, you want to bury the ears on each high rep. Try BB shrugs with different hand grips. Extra wide grip will also produce a good stretch and different muscle action to the traps. I've found that narrow grip (4-6") offer good results. Even one arm cleans/Hi-pull are excellent. A lot of the Olympic lifters have traps so deep and thick, you think you could hide your fist in them. Blame that on cleans and hi-pulls.
Might try Shrugs SS'ed with power cleans, 6-12 reps. Stay away from your normal shrug grip for awhile and do a set of wide grip shrugs SS'ed with a narrow grip shrug. Partial rep shrugs in a power rack are also outstanding.
If you have a middle trap structure, rather than a higher one, than the traps will not be that impressive from the front. From the rear they will be. If you a BB'er, a heavy set of traps will give the illusion of narrower shoulder width. Sorry to say but a neck strap will have little bearing on the traps themselves. Though it's always a good ides to work the neck hard and often. Good Luck.
