I get my students to start backfalling by sitting on the floor, legs out stretched.
You then bend one of your legs at the knee and fold it under the other leg. (make sense??).
I then ask the student to take hold of their outstretched leg at the ankle, and 'throw' that leg over (or wide of) the same shoulder. (left leg thrown over left shoulder, right over right).
Getting them to grab their ankle bends the body forward slightly before backward rolling and curls the back more than they usually would before attempting it.
Being already on the floor takes out the height of the fall and so alot of the danger, it also puts the momentum in the hands of the person practicing.
I then get them to slowly increase the height by getting them to go up on one knee.
When they first start out, the person training with them will take them to the point of throw and help them, by either letting go and allowing them to take the roll to the height and level they are comfortable with, or by controling them and talking them through the rounding of the back.
In our association back falling is very important for beggining students, we teach it in around week 3 and build it up from there.