First question: what's leftover pizza?
Okay, I admit that every once in awhile I may have a leftover slice or two. How I store them depends on where I got them.
If I ordered out or brought home leftovers from a neighborhood pizza place, the pizza box is the storage container of choice. Pizza boxes are designed to protect and preserve their contents for delivery or carryout, so why not for storage as well? Simply close the lid on your leftovers and slide the box into the refrigerator. There is even a new "green" pizza box being marketed these days that is kind of convenient. The lid is pre-perforated so that you can tear it into plate-size squares, ideal for serving your fresh or leftover pizza without dirtying dishes (which would then have to be washed) or using paper plates (more solid waste in the landfill.) I'm not fanatical about such things, but it sounds like a good idea.
One big drawback to storing your pizza in the box: storage space in your fridge. I don't know how many times I've had to "reengineer" a pizza box in a hotel or motel, because those little bitty refrigerators are way too small to accommodate a full-size pizza box. Same problem at home sometimes. If the refrigerator is pretty full which mine usually is it can be a pain to try to make room for a pizza box.
On to Plan B, the one I use for leftover homemade pizza: aluminum foil and wax paper. I used to just lift the whole wedge of leftover slices out of the box or off the pan, plop it onto a big piece of aluminum foil, and wrap it up. But I'd sometimes wind up with a wedge the size of an entire half of a pizza, which can still create a storage space issue. Then, one day I was using pieces of wax paper as separators for something and the light bulb came on. "Hm-m-m-m!" The next time I had leftover pizza to contend with, I removed the wedge of slices from the box, separated them into individual slices and stacked them up, using pieces of wax paper as separators to keep the slices from sticking together. Depending on your toppings, you can stack two to four slices this way, wrap them in aluminum foil or put them in plastic storage bags or containers, and refrigerate them. They'll take up a lot less room in the fridge and they'll still be easy to handle when it's time to warm them up.
Before addressing reheating techniques, allow me a couple of words about food safety. Oh so many people leave their leftover pizza out on the table or countertop overnight and oh so many people wind up sick as dogs because of it. The USDA and other food safety experts recommend leaving pizza out at room temperature for no more than two hours. After that, every extra hour that you leave it exposed to warmth, humidity, airborne pathogens, bugs, bacteria and other nasties increases the likelihood of a subsequent trip to the hospital and a visit with Mr. Stomach Pump. Or, at the very least, a lot more quality time in the bathroom. You might be able to stretch the exposure on plain cheese or veggie toppings a little, but anything with meat on it really needs to be refrigerated ASAP. And leftover pizza should only be a short-term resident in the refrigerator. Two to four days, max. Now, if the pizza is just the best that man ever created and you can't bear the thought of throwing any of the leftovers away, but know that you won't be able to eat them within two to four days, you can freeze leftover pizza as long as you do it within the two hour safety window. Leaving it out all night and then freezing it just means a delayed trip to the hospital.
Now, on to reheating. A lot of people like their leftover pizza cold. To each his own, I suppose. As long as it's been properly stored, no problem. But if you are going to reheat it, you really need to stay away from that ultimate convenience device the microwave oven.
In microwave cooking, radio waves penetrate the food and excite water and fat molecules. The food "cooks" from the inside out rather than from the outside in as it would in a conventional oven a process called "conduction," by the way. In a microwave, heat doesn't migrate toward the interior of the food via conduction. The heat is everywhere all at once because the molecules are all excited together. When you "cook" in a microwave, the whole heating process is different because you are actually "exciting atoms" rather than "conducting heat." This is fine if you're dealing with nice moist food that you want to remain nice and moist. But if you're talking about a thin and crispy pizza crust, microwaving it and exciting all those water molecules will just make it thin and soggy. That's why they make the nifty little "crisping" trays for pizzas that are sold as "microwaveable." The tray reacts to the microwave energy by becoming very hot. This exterior heat is then conducted by the tray, allowing the crust to become crispy as it would in a conventional oven.
So, just use a conventional oven to begin with. Or a toaster oven, if you prefer. If you have a baking stone - or a pizza stone, as they're often called warm it up in the oven (350 to 400 is fine if you're just reheating) and slap your leftover pizza on it for about five minutes. The stone absorbs the heat and redistributes it evenly to the surface of the crust, leaving you with a nice, evenly-heated crispy crust that is hard to tell from fresh. And, they do make baking stones that fit in toaster ovens. If you don't have a baking stone, try one of those perforated pizza pans that allows heat to circulate as the pizza bakes. If all else fails, just reheat it on the aluminum foil you stored it in. Just remember to remove the wax paper separators! And open the foil packages up. Leaving them closed produces much the same effect as microwaving does. The moisture turns to steam and steams your nice crispy crust to a limp soggy one, leaving you a little steamed when you try to eat it.
Leftover pizza can be even better than fresh sometimes. Something about the way everything marinates and blends, I suppose. But even the best leftover pizza will be bad unless it's properly stored.