DocRocket is partially correct. Self-rescue is important BUT OSHA harnesses ARE NOT the answer. He is correct on the practice too.
First off, OSHA fall protection systems (not just a stand alone harness) do not prevent suspension trauma as they have a lowering device and/or you are not alone. Second the front attachment is a recipe for disaster in a treestand with bows, guns in the way and you will end up lowering the attaching strap which results in more G's shock on a fall and definetly eliminate self recovery.
However, the new treestand harnesses are addressing that issue. They have wider straps, better buckles, better seat pans to put the harness under the butt and not on the legs/thighs.
Most are addresing what is called "suspension trauma" with attachable straps to address the suspension trauma issue. This allows the hunter to basically stand with his feet in an attachable stirrup and elimate the suspension trauma until he can self recover.
More importantly hunters need to understand where the attaching strap on the tree is located. ie not at waist level but above the head where IF you fall you go no lower than mid body on the stand. Most tree stands are setup improperly that results in the fall and inability to self recover. Improper placement of the attaching strap will significantly limit self recovery.
Hunter Safety Systems(and others) uses harness built into a easily put on vest with suspension trauma straps. Summit, others all now build those into their "aftermarket" systems. Forget the harness that comes with the treestand IMO. They are a total PITA to put on correctly and too many leave them at home or in the truck because of that.
If you are older or for some reason are limited in your ability to self recover here are a couple systems that address that issue.
Here are two systems that lower you to the ground on your command IF you cannot self recover.
http://hunterrescue.com/ has a complete self recovery system built in to lower you to the ground.
Mountaineer Sports has the Rescue One CDS system that does the same. It is easily repacked if you have to use it.
IMO one of the best, but not cheap. Then sow much is your life worth? less than the price of a medium qualilty scope?
http://www.youtube.com/watch?v=6K33QyuCu1w1. Check with your local state fish and game departments as most are now offering free specific tree stand safety courses that teach all of that and show you the new harnesses and recovery systems.
2. Setup your treestand in your yard about 2-3 ft off the ground and practice self recovery with your harness BEFORE you go to the woods. Learn what works, does not work and exactly how to make your stands and harness work together. Learn how to utilize the suspension relief straps and how to self recover.
BTW, 20 years as a safety specialist so I know OSHA fall protection systems and I am a certified trainer/hunter ed instructor for treestands.