Evergreens need regular trimming, both to maintain their shape and to keep healthy new growth coming. Left untrimmed, evergreens can develop a healthy outside layer of growth while the inside literally decays, drying up and putting the whole plant at risk of disease. It's not a complicated task if you have some know-how.
Examine the plant closely and remove any dead or diseased branches. Cut the branches off flush with the limb.
Cut off any crossed limbs, or limbs that are growing too close together. Again, cut them off at the base, flush with the trunk.
Trim the entire plant for shape, and to promote healthy new growth. When pruning small pine trees to maintain their Christmas tree shape, cut off any branches that stick out, making clean cuts. According to the Buckeye Gardening website, pine trees are best trimmed in May or June, during their growth period. Prune new branch growth midway to the point of old growth to allow for the setting of new terminal buds for the next year's growth. Junipers and other evergreen shrubs also should be kept in check. Thin the plant out and remove any errant or stray branches to force new inner growth.
Prune off a second top if it develops on a spruce or pine tree. Make sure there's only one strong central trunk; a second trunk will weaken the tree.