Remember to keep the root flare exposed when planting a tree or moving dirt around existing trees. The root flare area takes up oxygen for the tree.
A second thing to be careful about is adding too much weight (with soil or paving) over the root zone. Most roots grow in the top couple of feet of soil. They will extend farther than the trees canopy. A tree takes up the majority of its nutrients from the roots along the drip line (the edge of the canopy).