One of the challenges of throwing the backhand change is getting rid of the hump in the pitch — the tendency for the ball to go up slightly before dropping down on its way to the plate. The hump causes two big problems — it hangs it up high, where it's easier to hit, and it also makes it easier to see. 

The basic cure is to not let the ball go all the way to the bottom of the circle. Instead you want to finish flat, or parallel to the ground. Easier said than done.

Once cue I've found that works pretty well is to tell the pitcher to take her knuckles to the catcher's nose, or knuckles to nose for short. As she comes into the last part of the pitch she should take her knocking knuckles in a straight line as though she's trying to punch the catcher in the nose. The nose is generally about the height the hand will come through if it raises slightly above the bottom of the circle. You can even bring her up close to the catcher and show the difference between a straight in "punch" and an uppercut to the chin, which would require the hand to go all the way to the bottom of the circle.

It seems to be pretty effective, and is fun for the pitcher too.