There were a few slices left over, and upon returning with them wrapped in foil our gracious host dropped this knowledge on me:
When the pizza arrives, it's hot: On the top you have molten cheese; below that is thin sauce; then you have soft warm bread followed by that nice crunch of the crust. But as it sits there and time goes by, the cheese firms up, the sauce thickens down into the bread making it soggy, and the crust softens.
When you get home you need to reverse the process.
Get a nice skillet and heat it up. Put the pizza down. You can use butter or oil, whatever. Now think about it: As it heats up from below the crust will get crunchy again; the bread will heat up and soften; the sauce will rise out of the bread and thin; the cheese will heat up and melt. You'll be back where you started.
I followed this man's directions and there's no doubt he's right. This was the best reheated pizza I've ever had. How have I gone a lifetime without knowing this method?
(PS: Yes, I'm paraphrasing his words. I wish I could have recorded the original)