The best exercise for rear delts is descending sets of bent-over dumbbell flyes. Take a weight you can do 12 reps with. Then make 3 cheated reps. Without any rest drop down a weight and take new pair of dumbbells you able to do about 10 reps with. Continue to decrease a weight set by set doing 5-10 reps until your muscles give up. Rest about 2 minutes and make one more set, following the above mentioned technique. 2 sets are sufficient.