The behind-the-neck barbell press doesn't get the love it did in Arnold's heyday, as a number of trainers have decided it is either dangerous or not as effective as the barbell military press. Arnold, on the other hand, considered this movement an "old standby" and said it has remained one of his favorite shoulder movements throughout his competitive years.
So is this simply a case where Arnold's freaky strength and athleticism allow him to do what others can't? Not necessarily. Arnold knew that this movement worked best, and most safely, when performed in the strictest manner possible. No cheating here! Approach it with care, even at light weights, and do what is necessary to keep form tight.
For Arnold, this often meant performing the lift seated on an incline bench rather than standing, "since it makes the movement stricter," he wrote in "The Encyclopedia." He added in "The Education of a Bodybuilder" that "presses behind the neck should be done in front of a mirror, so you can correct yourself."
From this basis, simply take a medium-wide grip, keep the bar straight throughout the movement, and use a full range of motion, letting the bar come all the way down and pressing it all the way to lockout. Do it right, and you may find this to be as great an all-around shoulder developer as Arnold did.