I'm lucky i'm one of those guys with traps kinda built in from the start, but i like doing shrugs heavy on a machine we have at our gym that is actually for legs. It's one of those that you can put heavy weight on and... damn brain spasm... crap can't think of the name of it... but anyway it's got a handle on each side and they are connected so they don't move individually (damn i think i am dumbening... wait... dumbening isn't even a word). Anyhoo you can load it up with 45's and do shrugs easier than with dumbells or a smith machine. It'll let you go heavier than you could with just dumbells or a striaght bar is what i am saying.
Rich