The easiest way to learn the back kick seems to be to start in a swimming pool with no gear at all. Work on the mechanics with your bare feet -- extend your feet and legs, toes pointed, and then try to "scoop" water with the tops of your feet. You'll be suprised how quickly you end up moving backwards, and when you can do it very reliably, try it again (on the surface with a kickboard or something) with a set of fins. When you can swim laps backwards on the surface, then put on scuba gear and try it.
A few warnings: The back kick is very trim-dependent. If your feet are above your body, you won't move well backward, but more backward and up (the "shrimp dance").
It is very tempting to use the knees in the "scoop" maneuver (which is one of the reasons it's good to learn this on the surface). This results in the "humping the dog" kick.
This is an extremely difficult kick to learn in anything but a simple paddle fin with a reasonable degree of stiffness. Expect a great deal of frustration if you are using anything else.