Neck Flexion and Extension
If you have access to a neck machine, then get on that bad boy three to four times a week and work every angle on it for sets of 15-20. The problem is, most gyms don't have these around anymore. Don't worry, it's an easy fix.
Neck raises with a plate on your forehead (neck flexion) are absolute money for building a thick neck.
I used to do these every other day for several sets of 20 with a standard 45-pound plate on my head, but worked up to using three plates on my forehead for reps because I was a meathead. No need for you to do that; it really isn't needed.
For the neck extension work, I used a light resistance band because I hate trying to hold the plate on the back of my head. Stick with sets of 20 or more.
And no, I don't like the neck harness stuff. Doing neck extensions with a billion pounds is a bad idea.