A space heater is one of the easiest appliances to move, but the one real rule is to let it cool completely first. Oil-filled radiator heaters also contain sealed oil, so they should stay upright, while fan and ceramic heaters just need padding.
Pro tip: Pack the heater last and unpack it first, so it is not buried and forgotten in a warm season.
Watch out: Let the heater cool completely before packing, since the element and oil-filled fins retain heat and can burn you or scorch packing material.