HEAVY barbell shrugs with straps. I say with straps b/c I mean really HEAVY, and unless you have forearms like Popeye they'll probably fail before the much stronger traps will. Good luck.
Farmers holds(or walks)supersetted with shrugs(or something else that hits the traps hard..maybe cleans or hang cleans.) would hit your traps hard.I dont know if it would be best to do shrugs before,or after though.
I'd say shrugs are the number one trap builder. Just keep at it man. If you see your weights going up steadily, you can be sure your muscle size will increase as well. Be paitent... I too have learned this lesson.