deads and dynamic pulling (i.e. Olympic lifting and variations).
Instructions on performing the powershrug here http://www.elitefitness.com/forum/showpost.php?p=4497720&postcount=14 - it's the bottom link. You'll have to read a few posts because we kind of discuss it as a side line to the topic but don't ever do regular shrugs again. First, they suck and waste your time. Second, there is no exercise in the world like these for traps and upper back development. You will see major changes within a few weeks.