When working your rear deltoid muscles it is important to make sure you keep your form correct so that you actively engage these muscles throughout each working rep. The trick is to keep your arms slightly bent at all times throughout the move to avoid working other muscles which aren't a priority. Often I will see the arm start off bent and then towards the end of the move it will be straight and then bent again. This ultimately works more of the triceps than it will the rear deltoid muscle head. Incorrect technique can lead to insufficient results that will leave you feeling demotivated and frustrated.
Rear Delt Exercise 1
When it comes to your rear delts, one exercise for sure which is going to work them above anything else is using the cables. Standing rear delt flies are a perfect, isolation exercise which targets them like no other exercise. This is because a strict movement like this is only working that one muscle, make sure you keep your arms bent and not straighten them out at the end of the movement.
Rear Delt Exercise 2
On a back day when you are hitting rows into your workout, you will also be working the rear deltoid muscle. Seeing as you might have overdeveloped your front muscles (chest, shoulders) why not include a few sets of heavy rows which will also work on those rear deltoid muscles. There is no harm in doing some extra repetitions on exercises like this to help correct form and work on those weaker areas.
Rear Delt Exercise Tip 3
Instead of starting every shoulder workout like the last, with heavy shoulder presses and then side raises which will leave you totally gassed for rear deltoids, mix it up a little. So the next time you perform a shoulder routine do it in reverse order and start with rear delt exercises when you are fresh and will be more likely to work harder.
Rear Delt Exercise 4
Using the cable machine, attach a rope to it and have the pulley at head height. Take hold of the one hand at each end and pull this into your face, keeping the elbows high. It should look a lot like the picture below and will certainly work your rear deltoid muscles.
Including these exercises and really making an effort to work this muscle will generate a more balanced physique and can help reduce any injuries, you might have from overtraining anterior facing muscles. Give this a go and see how you feel after 4 weeks of making rear deltoids a priority when training and I'm sure you will see a difference in shape, size and muscular balance within this area.