I spent a lot of time choosing trees based on their size, leaf health, and how sturdy the trunks looked.
Because of the clay problem we planted the trees up high on mounds. This is to help with the drainage but also makes sure the tree is starting off with good soil until it has established itself
The most difficult was the feijoa because we had dug up the soil and when it rained it turned into a big pile of mush so we had to build a much bigger mound that would stay stable.
Sheet mulching (or no dig gardening) contains similar materials to a compost, and it builts healthy soil over the top of the existing ground.