Normally I would say think shoulder-in, but I have to teach shoulder-in first.
Thinking shoulder-in for a few steps/strides when the horse starts to fall-in, corrects the rider's weight and brings the horse back out and is usually the opposite of what the rider wants to do to stop a horse falling in. Usually I find a rider will turn their body towards the outside and try to push the horse out with the inside rein, weight going onto the inside seat-bone. Thinking shoulder-in brings the riders outside shoulder forward and levels the weight back up onto the outside seatbone, freeing the horse up to respond to the inside leg at the girth asking him to stay straight, or move back out.
So it depends on what's going wrong and whether you can ride shoulder-in and make yourself think shoulder-in, rather than trying to push your pony back out?
If your pony thinks you are asking her to fall in, which you could be doing, could make her frustrated when inadvertently asking her to fall in, you then try to push her back out and end up with her diving to the inside in her mistaken efforts to do what she thinks you want, rather than what you think you want.