To focus primarly on rear delts, seated bent-over lateral raises: sit on bench, chest to knees, thumbs down holding dumbbells, arms slightly bent, raise db to shoulder level out to sides... can also be performed on cables (bent over, hands cross at the start)... this one is more difficult to keep strict movement (no swaying)
or seated backward on pec machine pressing to the rear
forgot to mention... if you've never done these, this is an isolation exercise as opposed to a compound movement... proper form requires little weight on these to be effective
Rear delts are the only muscle group, that I actually prefer the machine, and or cables to free weights. I do not know why, it is the only muscle group that seems to respond better that way.
Heavy DB rows or heavy T-bar rows. I used to pound away rep after rep on bent over laterals and my rear delts never did put on any beef.
Think about how many guys have to train anterior (front) delts specifically. Very few. Because most people do so many for pressing exercises like shoulder presses and bench presses...they develop without any isolationary work. Same principle applies to rear delts.
Pound them with heavy compound movements and use the bent-over laterals only as supplements to the heavy stuff.
Try this movement out. Set the adjustable bench between 30 - 45 degrees and lay face down on it. Do lateral raises at this angle and it will give your delts a more rounded look when they are not flexed.