Pizza is a favourite of most people, and really, it isn't a terrible food. The things you have to look out for are fat and salt, and disproportionate parts (too thick of a crust, too must cheese).The tomato sauce offers the cancer fighting skills of lycopene, you get calcium and some protein from the cheese, and it is an opportunity to load vegetables onto something delicious. The trick is to choose thin crust, and multigrain / whole grain crust where available. If you really want to eat a healthier pizza (and probably actually enjoy it more) make it at home on either whole grain pitas or a whole grain crust from the grocery store (President's Choice Blue Menu makes a great one). Then, you can count on some fibre as well. Avoid most meats - sausages and pepperoni are just too high in fat to include in a meal that has so much cheese. Shrimp, tofu and chicken make great pizza toppings! Or, stick to just vegetables.
When I make pizza at home, I don't coat the crust in mozzarella. I prefer to sprinkle a light amount of feta or goat cheese, which has a stronger flavour and obviously saves a ton of calories and fat through using less.
When you order pizza, ask for half the cheese, thin and multigrain crust... and then here's the real trick... stick to TWO SLICES! And suddenly this junk food fits into a healthy lifestyle.