When your trees and shrubs start to look scraggly, you may want to give them a haircut. Pruning gives healthy branches room to grow. The plant's roots can nourish them better, since there's less to feed. A little pruning also helps keep diseases at bay. If in doubt, hire a professional to do the work for you.
When your trees are dormant and new buds haven't formed yet, get out your pruning sheers. The idea is to cut back growth that looks weak, ill formed, or overcrowded. Your healthy branches will be even healthier when they have more room and nutrients to grow.
If you want strong, healthy trees, don't prune them after their leaves have sprouted. Your trees have used up a lot of stored energy to push those leaves out, and they're too young to start replenishing the tree through photosynthesis. Pruning at this point will starve your trees. However, you can remove any sucker growth that you see around the base at any time.
If you have a shrub that blooms in the spring, prune it after the blooms have completely faded. If your shrubs bloom in the summer, do your pruning in late winter or early spring.