Summarize this article:

Use your shears or a hedge trimmer to cut unruly branches back from the base. This will prevent overgrown branches from sticking out too far and promote new side growth. If your shrub has multiple overgrown branches around the center, cut several back to the base with your trimmer. As you clear limbs, try to retain the plant's natural shape and keep the base the widest part.  Thinning out the center will help light reach the bottom branches. Avoid cutting center branches that aren't overgrown, as trimming too many main branches can harm shrubs. Inspect your shrub for sick-looking or broken branches, which must be removed to retain the plant's health. Cut away any rotten parts of the branch until you reach healthy wood. If part of a branch is healthy, you do not need to remove it in its entirety. Just remove as much of the dead or unhealthy section as possible.  Research common pests or diseases your shrub might suffer from and keep an eye out for early signs of damage. Cutting more than 1/3 of the plant makes it weaker and vulnerable to pests or diseases. If you feel like you might be over trimming the plant, smooth your work out as best as you can and finish trimming the shrub.

Summary:
Cut away the thickest overgrown branches first. Clear out some limbs near the center. Remove diseased or damaged branches or branch segments. Do not remove more than 1/3 of the shrub.