The deltoids can be a stubborn muscle group for some, if you manage to see some gains in this muscle group, they are often some of the first to plateau because they are a relatively smaller muscle group and can only take so much exertion before injury. Arnold used to use the lying lateral dumbbell raise to hit his middle delts, or medial deltoids to use their technical term. This is the area of the shoulder that (if looking at someone from the front) gives the width, one of many Arnold assets that helped set him apart. This particular type of lateral raise allowed for greater range of motion, maximum muscle tension and increased time under tension.
To perform this exercise try lying on your side on a slightly inclined bench holding a light dumbbell with your free hand. Start with the weight hanging above the floor and your arm across your body. Then raise your arm up laterally towards the ceiling and repeat for reps.
Reflex, (2015). Arnold Schwarzenegger: Lateral Progress. Retrieved 1st May, 2015, from http://www.flexonline.com/training/shoulders/arnold-schwarzenegger-lateral-progress