The withered blossoms should be carefully removed on a regular basis as otherwise they form seeds and thus consume nutrients which are important for development of the plants and new blossoms. By removing withered buds, you can achieve a second flowering with most flowers. Simultaneous follow-up fertilising also supports this process.
Pruning after blossoming also encourages plants to grow further in the next season. Pruning is normally carried out once a year. Correct cutting first of all requires the right tools. Garden shears should have sharp blades and produce a clean cut in order not to crush the plant. After all, clean, smooth cuts heal better. The more you prune, the more new shoots are produced. Thus the rule is that the gardener who prunes most consistently can expect the most blossoms the next year. But note: the later you cut, the softer and more frost-susceptible the new shoots are.
Ensure that you make clean cuts. Apply the shears at a 45° angle, as then the rain water can flow off more easily. Only cut in a period of bad weather or in cloudy weather. The exposed leaves and cuts will otherwise be damaged by sunburn.
However, there are slightly different cutting rules for pruning depending on the plant, which you should read in the relevant literature for your plant.