I struggled with medial delt work for a long time, and i feel im just now hitting them properly over the last 3 months.
The best thing ive found is to do lateral raises one arm at a time, strict form, slight bend in the elbow, laying sideways on a 45 degree angle bench. This is MUCH more difficult than a standing lateral raise. It eliminates the tendency to use a little body english to get the weights up, and starts the movement at an angle that is already applying tension to the muscle, so you dont lose tension at the bottom of every rep. I also feel its very important to not set the weight down on your thighs at the bottom of every rep, but instead stop just a couple inches above. Constant tension is the key.
My favorite way to do these is usually with a 30 lb DB for 12 reps on each side, and then drop to a 15 or 20 lb and hit both sides again immediately for another 12-15 reps. Nothing else i have ever done burns my delts out like this.
I also like behind the back cable lateral raises but find they irritate my left shoulder, so ill do these with a very light weight as a warmup or a finisher if my shoulders are feeling good.
for the record, while my shoulders are still one of my weakest points, i have seen significant improvements since i started doing lateral raises this way.