Standard exercises include upright rows, shoulder shrugs, front and back neck bridges. You also can buy a harness contraption that goes on your head and has a weight hook you can hang weights on add resistance to your head. They can be hard on your hair and ears, if so try wearing a wrestlers head gear under it. I've also seen a helmet like device with a dumbell bar sticking out of the top for adding weights. Or you can do the old "Charles Atlas" dynamic tension thing where you push on your head with your hand while moving it against that resistance. Most people find that their neck grows very quickly with resistance exercise, since it normally gets little exercise other than holding your head up.