Full harness or a simple strap but always tie yourself in!! To many guys get to qualify for the disabled hunts just by falling as little as a few feet.
I weara belt type set up where one belt goes around me but right up under the arms and the other goes around the tree. Not the best system, but some of the full vest harnesses seem to be bulky and wiegh to much.
I know you guys are alert and in tune the whole time you are in a tree but I have dosed a time or two and that little head bob that wakes you up can put the fear of god in you quickly.
I have heard of guys that wear a belt but wear it to low and have heard of instances that by doing that when they fall they arehanging upside down. You can pass out pretty quickly hanging upside down 5 feet down a tree. Just be careful!!