Originally Posted by Saddlebag
To protect the saddle, use a towel instead of any padding. When you set the saddle on as usual, grab the pommel and rock the saddle side to side. It will move back and will lock on. You will know this as it will be harder to rock. This is where the saddle fits best. Girth him up and go for a ride. With that style of saddle the height of both pommel and cantle should be the same.
Bear in mind a too-narrow saddle is likely to slide back further than the optimal position for one the correct width.
This is because it tries to find a position which allows it to sit as low as it can, in most cases nearer the base of the withers.
This sounds pretty far-fetched, because the horse generally appears to broaden behind the shoulder, but if you try it with a known too-narrow saddle you'll find it works in most cases :)