Rudolf Steiner devotes parts of various lectures to what he calls "the principle of spiritual economy." This is a very exceptional chapter of the story of reincarnation and karma. The etheric and astral bodies that have been transformed either through initiation, or simply through arduous individual work, are so valuable, that they are preserved in the spiritual world for the future use of others. It is comforting to know that the fruits of our works do not simply dissolve at death.
But Steiner does not stop and merely presenting this notion as theory: he discusses numerous individuals who actually received valuable etheric and astral bodies for the purpose of esoteric advancement in general, and the progress of Christianity specifically.
The number of passages in which he discusses this all-important concept is actually quite small. I was able to cut repetitive lectures, and thereby to present the heart of the issue very succinctly.