The first time I cooked with my now-husband, I was surprised to learn he had a collection of cast iron pans in an array of sizes. What impressed me most wasn't that he had a larger (and more well-seasoned) set of cast iron than me, but the smart way he stored his cast iron pans.
Separate Cast Iron with a Paper Towel
Instead of simply nesting cast iron pans one inside another, metal on metal, use a single layer of paper towel to separate the cookware to prevent friction. Follow the same rule of thumb if you stack other types of cookware on top of the cast iron, too.
Why This Is Important
We've talked about the importance of properly cleaning and maintaining cast iron, so think of this simple step as an extension of that. Separating nested cast iron pots and pans with a paper towel prevents scratches or damage to the inside of your cookware — and can absorb any moisture that might lead to rusting. You've put a lot of working into getting it seasoned, so giving it an extra layer of care will help keep it that way.
You certainly don't need to change out that piece of paper towel every time you use one of your cast iron pans (and if you have cardboard, that works as well). As long as the paper towel is clean and intact, it's still good. When that status changes, consider it a good time to swap in a fresh sheet.
Do you use paper towels in between your cast iron pans? Keep them on the counter? In the oven?