Lie prone on an incline bench with a dumbbell in each hand. Keeping the shoulder blades down, squeeze them back together and lift the straight arms out to the sides as high as possible, then return under control. Turning the thumbs down toward the floor will emphasize the posterior delt more and a more neutral grip will pull in more of the lateral delt.
 
Purpose
The incline bench rear delt fly trains the posterior and lateral delts as well as most of the upper back musculature to a lesser degree. Supporting the chest on a bench forces a stricter movement, which will keep the focus on the posterior delts.
 
Programming
Generally 10-20 reps are appropriate. Weights will typically be much lighter than expected because of the relative weakness of the posterior delts combined with the necessarily strict movement.