When trimming any hedge it is important to shorten the upper branches more than lower ones. The upper branches grow faster than the lower ones and if the same amount is trimmed, the lower part of the hedge will be narrower than the upper part. When this happens, the lower branches become shaded by the upper ones. The purpose of the leaves is to produce food by photosynthesis. When light becomes limiting, the lower leaves do not produce enough food and the plant drops them. When plenty of light reaches the lower branches, they grow full and thick. So always prune hedges so that they are slightly narrower at the top than at the bottom.
Arborvitae and other hedges often have large branches which stick out. Sometimes trimming off these branches leaves a big hole. If tucked in, they often just fall out again. However, you can tie them back in place using flexible plant ties. Most garden stores sell rolls of flexible green plastic which stretches. Ordinary twine will cut through the bark and damage the branch. The straggling branch can be tied to another branch inside the hedge to keep it from falling out again. If removing the branch would not leave a hole, simply cut it off.
For more information on pruning, go to naturalpruningnw.com and click on the pruning section under the How To Guide.