The type of pruning required will depend on the plant and what problem is to be corrected.
Perennials tend to grow into thick clumps of foliage that benefit from thinning. Removing some of the competing shoots will enliven the remaining growth, promote air circulation within the plant and allow more sunlight to reach the plant's center. In addition, there will simply be less succulent vegetation for pests to munch on.
Removing diseased plants during dry weather will help to minimize the spread of disease. Cut off the infected portion plus about 6 inches of healthy tissue, pruning back ONLY to a bud or main limb. Pruning too severely and leaving only a stub of bare wood will cause the once healthy portion to die and become a new pest entry point.