all good exercises j. i was going to suggest the lateral side raises standing, slight bend in the knee, bent over about 20 degrees with a good neutral spine. no need to go heavy on these, 15 pounds in each hand would be fine for the novice or even elite lifter. you control the weight, don't let the weight control you. it is an assistance exercise after all. whether you are a powerlifter, competing in strongman or just a recreational bodybuilder, no need to go heavy on these. now with that said, i hope you are focusing on your main compound lifts. overhead presses are going to build up the shoulders, anterior, medial and posterior heads just fine. yes your posterior heads may be lagging compared to the other two but over time they will catch up, you should be doing things like lateral raises and rotator cuff work to increase your work capacity or over all load capabilities in the OVERHEAD PRESS. also any horizontal rowing is really going to hit the posterior heads as well, just as any horizontal pushing is really stressing the anterior head. also you may be over working the rear delts trying to bring them up. who's to say. it'd be great if you could post your entire routine including legs, and we can work with you from there.