The preparation for underground work is deep spiritualization. As we peel an onion in preparation for its use, so God must "peel" from us what are mere words, sensations of our enjoyments in religion, in order to arrive at the reality of our faith. Jesus has told us "that whosoever will follow" Him will have to "take up their cross," and He, Himself, showed how heavy this cross can be. We have to be prepared for this.
We have to make the preparation now before we are imprisoned. In prison you lose everything. You are undressed and given a prisoner's suit. No more nice furniture, nice carpets or nice curtains. You do not have a wife or husband any more and you do not have your children. You do not have your library and you never see a flower. Nothing of what makes life pleasant remains. Nobody resists who has not renounced the pleasures of life beforehand.
I personally use an exercise. I live in the United States of America. Can you imagine what an American supermarket looks like? You find there many delicious things. I look at everything and say to myself, "I can go without this thing and that thing; this thing is very nice, but I can go without: this third thing I can go without, too." I visited the whole supermarket and did not spend one dollar. I had the joy of seeing many beautiful things and the second joy to know that I can go without.