Might suggest investing in a simple neck head strap. The neck area usually responds fairly quickly to direct exercise. Don't want to spend the bucks, than a folded long towel/belt can work well, with your own resistance. The muscles of the neck are short ranged. As are most other short range muscle groups (abs/calves), very powerful when fully developed. And can handle a lot of weight. The neck seems to respond better with a higher rep pattern (10-12, with some guys going higher).
If your somewhat adventurous, than wrestlers bridges are among the best movement, though not for everyone. Some haves bridged with over 200+ as a part of regular training. Using only bwt resistance works well with many people, but one thing to avoid is any ultimate stretch to the neck when training. Higher raised shrugs can affect the neck muscles, because the neck and traps were designed to function together. The old wrestler headlock resistance can work very well also.
If you have a training partner, that having him apply resistance can work very well, usually you may only need that long folded towel or neck strap. Some have hooked up a neck strap to cable machines with good results also.
Good Luck.