ThinkGeek is selling 24-packs of these Mama Fresco's Pizza Saver storage bags for $3 (the cheapest price I could find, even Walmart and Amazon are selling them in 12-packs for $6). They're 10-inch x 11-inch triangular food storage bags for leftover pizza slices (previously: the reusable heavy plastic Pizza Pouch, which can even be worn as a necklace so people know you're a pizza loving maniac). Alternatively, finish all the pizza the night you order it like a normal person with zero portion control. "Every pizza is a personal pizza if you try hard and believe in yourself." Now that's a winning attitude.
Keep going for a couple more shots.
Thanks to Luc, who just wraps his leftover pizza in Glad Press'n Seal wrap because he's a rich person who can afford Glad Press'n Seal. I reuse aluminum foil.