when i train neck i like to use either my neck harness or hang my head off the end of the weight bench(back on the bench) , put 50-60lbs on my forehead and do "neck crunches". it looks goofy as hell but this winter when i was doing this 3 times a week my neck got pretty huge.
i think that the neck can be worked more frequently than other body parts because its carrying that big ass head around all day. in other words, i dont think you can overtrain it.
the only time i wouldnt train neck is the day after i do power cleans or deads because your traps come into play when you work the neck.