The Basics

1) Thaw. Remove packaging and defrost in microwave (using the defrost setting or 50% reduced power) until the center is soft enough to easily absorb a fork.  Approximately 1-2 minutes for slices, and 5-6 minutes for full pies.  

Don't worry... this won't harm your pizza!  In fact, it's a critical step when it comes to reheating thicker pizzas. Otherwise, the center and edges won't cook evenly.

2) Decide.  If you like a softer crust, place the pizza on a sheet pan or tinfoil. If you prefer a crunchier crust, place your pizza directly on the oven rack.

This is about exposing/protecting the undercarriage of the pizza from the hot, toasty oven air.

3) Reheat. Place on middle rack and heat at 350.  ~10 minutes for slices, and ~15-20 minutes for full pies.

Cook times will vary since all ovens are different. Better to remove too early and put back in, than too late!

Expert-level

1) Turn. Most ovens are hotter towards the back, so spin your pizza anytime after halfway.

2) Trim.  If cheese along the outside is turning dark but center is still cold, trim edges and continue heating. Great trick when hungry!

3) Toast.  Slice and serve, then put any remainder back in oven to keep warm.  Turn off oven to prevent overcooking.

4) Read more here.