I was asked to post this tip from another thread. heat up a non-stick griddle pan, quite hot. place a couple of slices of pizza face down for about 30/40 seconds. turn over and cook the base for about 1.5/2mins. The results are pretty good. the base is crispy and the toppings hot.
We reheated pizza in this way (well with a normal frying pan ... I may get in trouble if I want to pack a griddle pan!) when we were camping in the summer ... worked a treat!
Sort of on topic- I finally figured out how to successfully reheat French fries. Just put a little oil in a sauce pan. (just enough to cover the bottom) Heat it up and dump the cold soggy fries in for a minute or two. Keep a lid on it and shake it about like you're making popcorn. Good as new.