Some thoughts on shoulder rests, and the poly-pad in particular:
Most players adjust instantly (and with delight) to the poly-pad, finding it to have a comfortable fit that facilitates holding the instrument with correct posture. Others miss the “security” of the relatively inflexible shoulder rest they have been using, because they have become overly dependent on shoulder support. In either case, after a period of using the softer sponge shoulder rest instead of a rigid one, all are likely to find themselves less inclined to press their shoulder against the shoulder rest (for this turns out to be counterproductive). It is important to understand that a shoulder rest is intended only to provide stability by filling space between shoulder and instrument; it is not to be used to assist “clamping” the instrument between shoulder and chin/jaw.
The poly-pad has also been found to be a helpful intermediate step for players wishing to wean themselves from using a shoulder rest altogether. Although I use my poly-pad practically all the time, when I pick up another viola to try it out, I’d much rather play it with no shoulder rest (and am quite comfortable doing so) than with a rigid bar-type rest. In my pre-poly-pad days, going from shoulder rest to none was a really big deal – not so now!
One more thought: All players, especially tall ones with long arms, may need to be reminded to keep the scroll pointed sufficiently to the left that the instrument is over the shoulder, not the chest. As the space between shoulder and jaw is obviously less than the space between chest and chin, a lower shoulder rest may possibly be used. Holding the instrument in this orientation can also improve bowing (the right arm is more open) and general left-hand technique, especially shifting (the left arm need not be so twisted and pulled so far in front of the body).