Push with your fingers on the rear panel behind the putty knife to minimize bending. Slowly flex the putty knife, as shown in the picture, to ensure that most of the metal tabs on this side of the iPod are disengaged.

The theory behind this method is, rather than attempting to not bend the rear panel at all, to bend it in a favorable manner that allows you to easily restore it later. Therefore, any bend in the sides of the rear panel should be drawing the lip of the rear panel away from the iPod, rather than pushing out on the curved surface. This method also disengages as many of the side clips as possible.