When digging the hole for planting it is beneficial to break up the soil 10-15cm deeper than where the bottom of the plant will be. This soil loosening will encourage the roots to go deeper, giving the plant better stability and more moisture availability during the dry summer period.
Slow release fertiliser should be added (especially beneficial on hard sites or in poor soils), placed around the sides of the rootball when the hole is half filled. The tree should be planted to the same depth as it was in the container or nursery bed. Poplars and willows however can be planted deeper without danger of stem rot. Fill in the hole and gently heel in with your boot around the tree.
A thorough soaking with water will also help the plant adjust to its new location. In hot weather it is better to plant early or late in the day.