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, he used a single paper towel to separate each piece of cookware to prevent friction. This trick is also worth stealing if you stack other types of cookware on top of the cast iron, too. (Oh, and you can use TWO paper towels if you're trying to be extra careful.)
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.
And you don't even need to change out that paper towel every time you use one of your cast iron pans. As long as the paper towel is clean and intact, it's still good. (and if you have pieces of cardboard, that will work just the same way.) When the paper towel gets dirty or dingy, then it may be a good time to consider swapping in a fresh sheet.
Do you use paper towels in between your cast iron pans? And where do you store them? On the counter, in the oven, or somewhere else?