Scale it to the room
The commonest mistake is trim sized for a magazine rather than the ceiling it is in. Heavy crown in an eight-foot room makes it feel lower, not grander. Taller baseboards almost always read well; crown depends entirely on ceiling height and whether the ceiling is level enough to take it.
Material
MDF takes paint beautifully, costs less and is stable, and it is the right choice for painted trim in a conditioned house. It is the wrong choice anywhere it might get wet, including a bathroom baseboard next to a shower. Poplar or pine where it needs to be robust; hardwood where it will be stained.
Where crown molding installation goes wrong
Ceilings that are not flat. Crown follows the plane it is fixed to, so a ceiling that dips shows as a widening shadow line along the wall, and no amount of caulk hides it. On an older house the honest sequence is to check the ceiling first and either accept the variation, scribe the moulding to it, or level the plane before any trim goes up.
The details
Coped inside corners rather than mitred, so the joint stays closed when the house moves. Scribed to the wall where the wall is not straight, because in a forty-year-old house it will not be.