Here's a slice reheating tip that's sure to catch some flack, but I tried it on a whim and found it works very well if I do it right. The key to success here is a fully preheated stone, bricks, or whatever. Take your 2-day-old, past-its-prime slice and do the following:
1) Turn it bottom-up and pass it under running water in the sink. Don't linger, but get it fairly wet.
2) Set it on a paper towel.
3) This part is up to you. I find that after a day or two, there's a flavor deficit. I grate a bit of pecorino over the slice and/or even sprinkle some salt. Then I drizzle a little evoo on it.
4) Throw it on the hot deck. My oven gets hot; so the water creates a sizzle. It won't be soggy when you're done. That little bit of steam revitalizes the slice the same way it can a piece of stale bread. Keep it in until the cheese is bubbling and the oil has been fully incorporated into the proceedings. The water also lets you leave it in those extra seconds without incinerating the slice.
5) If the oven's hot and it's the right slice, you'll get a nice crisp-but-not-tough slice in the end.