The hole when dug should be twice as wide as the size of container or root ball, but no deeper, as this may cause burying of the crown as the soil settles.  Simply scoring the bottom of the planting hole will aid in directing roots to the deeper subsoil.  Whether in a root ball or a container, you want the soil level in the container or on top of the root ball to be flush with the plane of your existing soil. The important element is to not bury the original crown of the plant.  That must be above the soil level to keep it from rotting.

When planting, it maybe necessary to amend the soil in order to increase moisture retention or to increase drainage.  In order to help the common clay soil hold water as well as air, it is good to add some topsoil, peat moss, or organic compost to the hole. When amending, use at most a fifty-fifty mix of the backfill soil and the additional soil.  If you need to increase drainage, you can raise the soil at least several inches from the surrounding soil by making a raised bed.  You can also add some pine soil conditioner, vermiculite, perlite, or gypsum to help with drainage.