Spring pruning consists of removing dead flower heads and dead and weak stems. If the bush is overcrowded, some of the old stems can be cut right out, allowing new growth to take its place.
Don´t prune in autumn, as the old flower heads should be left on all winter to give frost protection to the delicate new growth just below. They look fantastic when their brown, papery domes are covered with frost. Pruning is not essential but may be done each spring as new shoots appear.
Don´t cut the brown stems off in the winter. The brown stems are what next year´s blooms will grow on. You may shape the plant by shortening them, taking off an inch or so on smaller plants and three to five inches on larger plants. Always cut just above a node (joint). They seem to like this "haircut". The next winter the brown stems will turn a whiter colour and start to peel off in papery layers. This is normal and healthy.
On a very full mature plant, you can remove some of the brown stems completely to encourage fewer, but larger blooms the next year. Leave the stems alone for more, smaller blooms. Paniculata varieties are better not to be pruned unless the branches get overcrowded or untidy, in which case they should be taken out at the base in late spring after the danger of frost has passed. Alternatively, hard prune in spring to keep compact and to produce enormous flower heads.