If you can manage to grab a large object, such as a plank or a piece of rafter, you'll greatly increase your chances of survival. The object will absorb some of the impact when you land, taking a little stress off of your bones. If you're falling next to a building, or off a cliff in the wilderness, do your best to break your fall into segments by hitting a ledge, a lower cliff, a tree, or another object. This breaks up the momentum of your fall and divides it into several shorter falls, which gives you a much better chance of surviving. If your knees and elbows are locked and your muscles are rigid, the impact of the fall will do more harm to your vital organs. Don't hold yourself stiff. Do your best to relax your body so that when you hit the ground your body is more easily able to take the impact.  One way to remain (relatively) calm is to focus on performing the steps that lead to greater survival rates. Stay aware of your body, moving your arms and legs to make sure they aren't locked. Possibly nothing is more important to surviving a fall (or simpler to do) than bending your knees. Research has shown that having one's knees bent at impact can reduce the magnitude of impact forces 36-fold. However, do not bend them too far - only keep a slight bend so they are not locked. No matter what height you fall from, you should always try to land on your feet.  Landing feet-first concentrates the impact force on a small area, allowing your feet and legs to absorb the worst of the blow.  If you are in any other position, try to right yourself before you hit the ground.  Fortunately, attaining the feet-first position seems to be an instinctive reaction. Keep your feet and legs tightly together so that both your feet hit the ground at the same time. Land on the balls of your feet. Point your toes slightly down before impact so that you will land on the balls of your feet. This will allow your lower body to more effectively absorb the impact. Once you land on your feet, you will fall either to the side, front or back. Try to avoid falling on your back. Falling to the side is statistically best. If you can't manage that, try to fall forward instead, breaking your fall with your arms. When you fall from a great height onto land, you will usually bounce. Some people who survive the initial impact (often with a feet-first landing) suffer a fatal injury on their second impact. You will most likely be unconscious when you bounce. Cover your head with your arms by placing your arms on the sides of your head with your elbows facing forward (and projecting in front of your face), and your fingers laced behind your head or neck.  This covers a large portion of your head. With all the adrenaline flowing in response to your flight, you may not even feel injured upon landing.  Even if you are not visibly injured, you may have sustained fractures or internal injuries that must be treated immediately.  No matter how you're feeling, get to a hospital as quickly as possible.

Summary:
Grab an object on your way down. Try to break your fall into parts. Relax your body. Bend your knees. Land feet-first. Try to fall to the side. Protect your head for when you bounce. Get medical help immediately.