What do you incorporate into a stuffed animal so that it doesn't permanently deform?
Compartmentalization

One way to prevent displacement is by making sure the stuffing cannot move anywhere. If all basic shapes (limbs, head, torso, &amp;c.) are separated from the others by an internal wall or membrane, you will prevent the filling from moving outside their limited volume.

Use pre-shaped filling

You can alternatively adapt the stuffing to the final shape of the volume it will occupy.
You will probably have to adjust your technique a little, since this requires adapting the shell fabric around the stuffing, and sewing it in place.
This will have consequences for the tactile qualities of the stuffed toy.