Here are a couple of neck exercises that I and a couple of the guys I train do:
Basic Wrestlers Bridge-lay on your back and do a gymnastics bridge but instead of supporting yur self with your arms, support yourself with your neck. Put the top of your head on the floor and roll back on your head so that your nose touches the mat (or try to, it is hard and takes time to develop the proper flexibility).
Basic Wrestlers Front bridge-Kneel on the ground. Place you hands face down on the mat in front of you and place your forehead on the mat. Extend your legs behind you so that your body weight is supported by your neck. Roll your neck in circles.
Wall Exercise-Stand facing a wall. Using a towel to cushion your face, place your forehead against the wall. Step back a bit and let your body weight be supported by your neck. The more parallel to the floor you are the more pressure that is on your neck. You can perform a variation of the same exercise for the back and side neck muscles also.
Bench Exercise-lay on a weight bench or other flat surface like a table. Have your back supported but your head and neck off of the table. Place a weight (we started off with a 25lb'er) on your forehead, use a towel to cushion your head. Your goal is to keep your chin against your chest, to support the weight with the muscles of your neck. The goal here is to hold the weight for a full minute for three to five sets. When you can support the weight for a full minute you increase the weight. This is a really great exercise that was taught to me by the "Hooker" Tony Cecchine.
You could always buy a head/neck harness and do the standard front, back, side to side lifts