My secret for accomplishing such isolation is to do nearly all of my shoulder exercises seated, to prevent any rocking or momentum in any other part of my body. This also fixes the angle of my body, so that each repetition stays focused on the precise part of the deltoid that I want to hit. To further ensure that the stress remains in my delts, it’s also necessary to keep my chest out and my head aligned with my upper body; if I let my head drop forward, the stress can tend to shift from my deltoids to my traps.
