First you gotta wrap up the leftovers. It doesn't matter if its foil or wrap, but you don't want the leftovers to get that hardboard quality from being stored in the box (or – god forbid – even open on a cardboard disc in the fridge acquiring the cumulative flavor funk of your fridge.
DO NOT use a microwave. I'll concede it is faster, but it leaves the pizza kinda limp, and as it cools the crust gets a sort of toughened quality (and eventually hard).
While I cannot say I invented it, I've been doing this since I was a teen with great results.
The method you have to try is using a frying pan, set to a medium low heat. Place the slices crust-side down, and place a lid (I have the tempered glass lid pans which is nice). Heat em until the cheese starts to melt on top and the top gets hot. The bottom crust will be crispy (and keep an eye on it-- you don't want to burn it). For my oven its abut 15 minutes (on electric burner set to 3.5 /9).
