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Pruning evergreens for shape or size should be done seasonally, typically during the dormant period or just before the major growth period. However, if you see a branch that is broken, has no foliage, or appears diseased, you can and should remove it right away, no matter the season. Contact a local agricultural extension office, a tree nursery, or an arborist for information on evergreen diseases to look out for in your area. Before and after using your pruners to snip away at evergreen branches, you should wipe them down with a rag moistened with rubbing alcohol. This will remove dirt and sap, and also sanitize them so that you don’t inadvertently transfer any evergreen diseases.  To be extra safe, also wipe down your pruners when moving from one tree to the next. When you’re cutting branches that may be diseased, you should wipe down your pruners after every snip. In most cases, a good pair of loppers or hand pruners will be all you need to prune evergreens. However, you may also want to use a hand saw (for branches over 1 in (2.5 cm) thick), or handheld hedge clippers for shearing. If so, clean them with rubbing alcohol in the same manner as your pruners. Always sanitize tools before and after using them, whenever you switch trees/plants, or after you cut any part of a plant that may be diseased. At the point where a branch emerges from a trunk or thicker limb, you’ll see a slightly thicker ring known as the branch collar. Instead of cutting the branch completely flush to the trunk or limb, leave this small collar behind when pruning. Doing so will cause less damage to the trunk and make it less susceptible to diseases. If you’re pruning a branch that’s at least 1 in (2.5 cm) thick, trying to snip or cut it at the collar may cause tears in the bark and damage to the trunk or limb. Instead, make 3 cuts:  Snip or saw the bottom half of the branch, about 12 in (30 cm) out from the collar. Snip or saw the top half of the branch, about 1 in (2.5 cm) further out from the first cut. This will cause the branch to snap between the cuts and break off. Cut off the remaining 12 inches (30 cm) of branch at the collar. This will reduce the weight of the branch significantly, so the bark will be less likely to peel or tear. Most types of evergreen trees have a branch known as the leader that extends vertically upwards—this is the branch where you’d place the star on a Christmas tree, for instance. If the leader becomes broken or damaged, you can train a nearby branch to take its place:  If possible, trim the damaged leader to about 2 inches (5.1 cm) above its lowest side shoot (i.e, smaller branch that extends out from the leader). Locate a healthy branch near the leader, bend it upward, and tie it to the old leader and trunk with rope to keep it vertical. If necessary, create a splint with a scrap of wood and tie it in place. Remove the rope or splint after a year and see if the branch holds its vertical position as the new leader. If not, tie it up for another year. If you prune an evergreen late in the growing season, you might inadvertently spur new growth just as the typical dormant season begins. Especially in colder climates, this fragile new growth will not survive, and its death may cause more widespread damage to the evergreen tree.  Seasonal pruning only involves trimming for size or shape, though. You can remove dead, diseased, or damaged branches at any time. In most cases, spring—and often early spring—is the best time to prune evergreens. Late fall pruning is also usually OK, and there is often a period in midsummer when evergreens go semi-dormant—you can also do some pruning then.

Summary:
Cut away dead, diseased, or damaged branches promptly. Sanitize your pruners before and during trimmings. Sanitize any other tools you use in the same fashion. Leave the “branch collar” behind when you remove a branch. Remove thick branches by making 3 separate cuts. Train a new “leader” if the current one is damaged. Limit your pruning during the late summer and early fall.