Emphasizing the middle deltoids:
shoulder press
I wouldn't recommend doing these behind the neck as this puts a great deal of unnecessary strain on the shoulder girdle. Tends to work more of the front delts, and the middle delts some. Rear delt work must not be neglected! Obvious variations of this exercise are using barbells or dumbbells.
Using dumbbells uses more supportive muscle and is a great exercise to use every once in a while for the same reasons that doing dumbbell benching is so good. Be careful to not let your arm wander to far back, as this is probably where most shoulder dislocations occur!
upright rows
Grab a barbell more than shoulder width apart, and raise the bar until your upper arm is parallel with the floor. This is usually done with a narrow grip, which can put strain on the wrists, and also calls into play triceps and traps. This exercise targets mostly just the middle deltoid head.
lateral raises
Usually performed standing, knees slightly bent, slightly leaning forward ( to target the rear delts more ), elbows slightly bent. Raise the weight as fast as you can, and lower slowly and under control