Originally Posted by Island Horselover
Make the backing up your idea. As soon as she starts make her back up, not just a couple of steps, go 10 steps or more and then make her go forward right away. You might have to do that quiet a few times but she probably will think, well when I back up I just have to do it even more.... it helped with my Morgan mare who can be REALLY stubborn, it worked out for us though.... good luck!
The gelding I had previously came to me with this exact bad habit. As soon as I asked for more than he wanted to do, he'd put his head down, ignore my leg, and back up. I did the exact same thing as Island suggested. I said "okay, you want to back up? Well, we're going to back up all the way across the arena." They figure out real quick that its much easier to just go forward than have to back longer than their comfort zone.