Arnold's two undeniably great books, "The Education of a Bodybuilder," and "The Encyclopedia of Modern Bodybuilding" aren't just classic because they show how he and other great bodybuilder trained. They've stayed in print and in use because he's equally revealing about what he did wrong in his training for years, and how he had to change his approach in order to become the greatest of all time.
When it came to the stable shoulder movement of lateral raises, Arnold is insistent that we learn from the error of his ways. "For many years I was doing my lateral raises wrong—with the thumbs up—because I saw them done that way in pictures, in magazines," he writes in "The Education of a Bodybuilder. "I could never figure out why my rear deltoids didn't grow. Then once, I was experimenting at home and I found that by turning the wrist to the side and making it straight, like a horizontal fist, with the thumb pointed toward the front and the weight straight, I got sore in my rear deltoid area. The more I turned the little finger up, the more the strain would go into the rear deltoid."
He also advises staring in a slightly bent-over position so that you don't "borrow force from other muscles." If you see the way most lifters perform lateral raises, you know exactly what he means.
Are your shoulders built like a cannonball all the way around, or are they lopsided like a jack-o-lantern that's been sitting on the front porch for too long? Take a look in the mirror or ask someone you trust for a frank appraisal. You know that's what Arnold would do. Then he'd do something about it.