The independence of arms and body and legs and body is also important, but I never had a problem with that.
Keeping my upper body from turning when I turned my head to look over my shoulder proved to be very difficult. Every time I skated backwards and turned my head, my shoulders went with me.
I was turning my head like Angelina Jolie in the picture below. What's notable about this picture? The chin is pressing down towards the shoulder.
|I'm not this pretty|
This 'look down over the shoulder' appears to be the norm as I went through hundreds of pictures before I found this:
|But I am this cute!|
Here's a better picture:
See, chin up and tilted. This works for me.
I didn't experiment much or anything, I just stumbled on it. One day I got my chin up and got stable going backwards instantly. If you're having trouble checking behind you while you skate backwards, let me know if this tip works for you, 'cause I have no idea why it works.