Heavy back rowing motions like DB rows, T-Bar rows, BB rows, etc...then finish them off with bent-over laterals with DB's or cables (I prefer cables for the constant resistance.) Posterior delts are hard to isolate. Practice flexing them between sets so you can improve mental control of them.
lay on an incline bench prone. take about a 10-15 lb dbell and practice isolating your rear delts. you'll get the feeling of when you are hitting them by doing some rear delt raises. the light weight ensures you dont swing the weight. if you take about 3 seconds each direction. pos. and neg. you will feel it, and they will be wrecked.