Falling is an art form; the more you do it the better you get at it. Although you cant control the fact that you aren't going to land directly on your feet when you fall you definitely can control what position you will land in, what part of your body you will land on, and how you will maneuver your body after the initial impact. Typically you want to take as much of the impact with your legs as you possibly can (If you can at all) then tuck your shoulders, land on your back / shoulder area while lifting your head up and away from the ground, and role it out as much as you can. The aim of the game is to increase the time it takes for your body to come to a complete stop consequently reducing the average force needed to make your body come to that stop, and in the process taking the impact with the parts of your body that can take it the best. Although using your arms to help stop you might seem like a good idea, I would say avoid using your arms unless you absolutely need to keep from whacking your face on a rail or something; your arms aren't really made for taking impacts and can easily be sprained / broken.
Key Points-
-Take as much of the impact with your legs (If possible)
-Land on Back/ Shoulder area
-Tuck n Roll (If falling forward or to the side do a misty roll, if falling backwards do a rodeo roll)
-Keep your head up and away from the ground (Protect your noggin)
-Avoid using arms
Of course this doesn't work in every situation especially for when your falling directly on top of a rail, but this is the most general technique that can be used for most falls.
