I find as I grow older, and loose manual dexterity, I find it helps to work with my hands, not my arms. This means holding the model in my left hand and the part I am attaching in the right hand, or tweezers and part in my right hand. Then I need to touch my hands together lightly.
It does indeed help to touch the left hand to the workbench to stabilize, even just very gently. I sometimes stick out fourth or fifth finger to touch bench. You want to minimized the total length of the muscle chain between model and part. So sometimes I use the little fingers to touch each other. These fingers do not generally get called on to do fine work, but it only takes a few hours and they will catch on.