There are two ways to eat leftover pizza (and have it taste good). Most of us guys know the first — cold, straight out of the box where it was left last night.
The second way tends to be a bit more acceptable to the wife.
Set your toaster oven (or oven) to around 500 degrees and let it heat up. Once hot, put the pizza slices right on the rack (putting a cookie sheet underneath to catch any cheese dripping is highly recommended). Cook until the cheese is bubbly. This won’t take long, 5-10 minutes, so keep an eye on it.
It is not quite as good as fresh pizza, but it’s close!