Most young trees that fail were planted wrong, usually too deep or in a hole that's too small. Get the planting right and the tree largely takes care of itself.
Dig wide, not deep
Dig a hole two to three times as wide as the root ball but no deeper than the root ball is tall. The widened, loosened soil lets roots spread; a too-deep hole settles and buries the trunk, which rots the bark.
Set it at the right depth
The root flare — where the trunk widens into the roots — should sit at or slightly above ground level, never buried. Loosen circling roots on container trees and remove wire and burlap from the top of balled trees.
Backfill and water
Backfill with the native soil (heavy amendments can discourage roots from spreading), firm gently, and water deeply. Mulch a ring around the tree — but keep mulch off the trunk — and water regularly through the first couple of seasons.