Stacking pans on top of one another can scratch their nonstick surfaces. Sandwiching paper plates between them, however, will protect their finishes.
Cheap but Effective
Inexpensive paper plates are the best choice because they have more give than the sturdier kind, thus allowing the pans to stack more easily.
The Right Tools
Metal utensils can scratch a nonstick finish, so it's best to use heatproof plastic utensils on nonstick pans.