if you are familiar with a gymnastic bridge then try doing it without your hands. practice raising your back off the floor with just your neck and then moving slowly back down towards the floor. i saw mike tyson doing this exercise and it will definitely help you develop your neck.
or:
you could buy one of those head straps that come with a chain. you hang the weight off the chain and then just look down slowly(moving the head, not eyes), then look back up.