Mike Tyson used to work his neck once a week. of course a boxer would have a much greater need for a strong neck than most, due to the amount of jabs they take to the face, the strong neck muscles prevented there heads from bouncing after each punch so they could keep their eyes focused on their opponent. he used to lay on his back,
knees bent 90 degrees like he was doing standard crunches. with his feet in place, he would roll his entire
body weight back until the top of his head was on the mat, so most of his body weight was being supported by his neck. i tried it once, im lucky im still able to use my neck

i would never recommend anyone try this as it is a very difficult movement and without the proper training the risk of injury is very high, but if you happen to catch some of his training online or on an old video, check it out, it was really amazing