OK, it is best to prevent tree trunk wounds first. But if a tree trunk is wounded or injured there are some things you can do. Remember that a tree does a great job on containing and compartmentalizing trunk wounds and these treatments are not embraced by all tree professionals. The also can't do a lot of harm.
Remove dead and injured bark from around the wound with a sharp knife. Scribe wound in the shape of a vertical ellipse, if possible. Cut bark away from wound to form an interface of healthy wood and bark although this may enlarge the size of the wound.