The problem with rear delts is most clients (whom I have trained, trained with, or work with) don't get proper mind muscle connection. I would highly suggest a few things to help improve your rear delts.
1 - Put them first in your shoulder training
2 - Find exercises that give the best MMC
3 - Attack them from a fully stretched position
4 - Eliminate extra momentum
5 - Use Iso-Tension with a partner
A quick shoulder routine that I like to throw together for my clients
Rear Delt Cable Extensions or HS Rear Delt --> work up to a good set of 20/30 with isotension or drop set
DB Laterals or Front Raises (Medial/Side Delt)
HS Press or Smith Press (bring bar right to your chin and back up)
First engage the rear delts, then there is carry over from the other exercises which will stimulate them to a degree. Anyone can do an isolation for them but the premise is stimulation and proper form. Most people you see do rear delts either extend too far (HS/Cables) or use way too much momentum or "Swing" the weight. It is never in a controlled manner which is why they are so hard to engage.
People need to realize if you drop your ego and focus on feeling the muscle work you will see it grow (given proper nutrition, rest, and training). But then again most gym goers are there to number chase.
