Dumbbell shrugs are a great way to do it. I find them easier to do both shoulders at the same time, but when this happens it's best to work each shoulder one at a time. Really isolate the muscles. Train the underdeveloped side harder, and allow it to catch up.
Also, it should be noted that it's not unusual to have one side be bigger than the other. I'm pretty sure every guy here would say that their dominant arm is bigger than their non-dominant arm. (There's a masturbation joke in here somewhere.) We can notice it, but most people won't. Just keep training the underdeveloped side, and eventually it'll get closer.
