The Neck Helmet
If you want to look strong (not to mention also be strong) then you had better train your neck. This fellow, a football player at the University of Tennessee Martin, named Hunter Carter had some help from Mother Nature in that department but he also did quite a bit of work with a Neck Helmet shown here. You'll find him featured in the July, 1976 issue of Muscular Development Magazine in an article on neck training by Carl H. Giles.
Speaking from experience, a neck helmet trains the head and neck muscles in a unique manner and is an excellent choice though it is not without its disadvantages. To build the strongest possible neck a variety of equipment and techniques can and should be used, including plate-loaded neck machines, manual resistance, neck straps, jaw and teeth lifting, isometrics, and head stands (this list is by no means exhaustive). Keep in mind that building the strength and size of the neck is like developing any other muscle group, incorporate the overload principle, train progressively and recover properly and your collar size will inevitably increase.