So sorry about your water holding clay soil but sounds like you know all the tips and understand about poor drainage. I would loosen the soil all around the planting area, work some wood chips or other course organic matter into the backfill when adding a new shrub or tree and do not place any organic matter in the hole under the new plant. In your soil compost or other organic matter will hold too much moisture at the soil roots during the winter. Use a mulch around the roots after planting but do not let the mulch touch the bark of the new plant. You need to cover the top of the root ball as it will be slightly above grade. A heavy wood chip or bark mulch should stay in place. Another idea is to improve mounds of soil or bring in mounds of topsoil and just plant in these raised beds above the winter water table. Don't be discouraged. Clay soil is actually more fertile and you can create a beautiful landscape. Keep growing, Marianne Binetti