Being a stoker, I can relate to your problem. I have found that the easiest way for me to do it, is to stand Vertical, and tilted forward. The Arms I keep very loose, and still stay upright. I let the pilot put his body and arm movement into the side to side movement of the Tandem. This means that I have to lower my torso by tilting forward. However- On my spinning classes, They were teaching us a method of OOS pedalling that works when I am on my solo. The bike does not sway as much and seems to be a lot smoother. Very easy to learn this on a Bike that is tied to the floor, but how to get my pilot to learn it is beyond me.