A lovely specimen tree, arborvitae (Thuja spp.) often makes itself useful as an attractive, living screen that provides privacy for family activities. Most species of this loosely branched North American native grow in United States Department of Agriculture hardiness zones 4 through 7. Once established, arborvitae mostly cares for itself. It has no need for regular pruning, but it does benefit from an occasional trim.
Remove branches in early spring that are unusually long or that otherwise don't contribute to a pleasing, conical form. Use handheld pruners or lopping shears, and cut almost to the main leader or to outward- and upward-facing growth. You can also do this task in December, and use the cuttings as Christmas greens.
Cut out dead and diseased plant parts any time you find them. Cut to healthy growth, or all the way to the center of the plant. Look for insect and drought damage in summer; wind and snow damage in early spring.
Thin out branches where individual trees touch each other in hedges or windbreaks. In keeping with the purpose of these plantings, it's okay for the trees to touch or gently "knit together." However, make sure the branches are not so crowded that they deprive each other of light and air. Cut the occasional branch -- maybe just two or four on each side -- all the way to the trunk. Often, a branch will tell you it needs trimming by turning partially brown or leafless.