That, or, if adding is difficult, don't subtract as much. Digging the planting hole only as deep (or not quite as deep) as the root ball that's going in will help to combat overly deep settling. I like to do mine three times the width, though; that allows for replacing native soil directly against the root ball, with room left over near the edges for adding compost. Settling near the edges is a good thing; a slight reservoir develops to catch and hold a bit of extra water.
