The posterior or rear delts are notoriously hard to target, leaving many lifters with underdeveloped physiques. If that's you, try this:
Brace your non-working arm on your thigh and keep your torso motionless. Rather than standing right beside the sleeve of the bar, orient your body slightly toward it. This will help you better connect with the posterior deltoid and de-emphasize the rhomboids and middle trapezius.
If you find yourself lifting your trunk, allow more bend in your elbow.