Avoid the wet stuff.
Window fog builds because the air inside your car is moist. When temperatures drop, the air an not hold the moisture and much of it deposits on your windows. So do not bring any new moisture in. Damp umbrellas or sweaty gym clothes go in the trunk. And if there is snow on the ground, sit down and smack your boots together before you swing your legs in the car.
Wash your windows.
You clean the outside of your windows, but you should also give the inside a regular spray and wipe. Dirt gives moisture a base to settle on, making it easier for fog to form.
Do not smear the glass.
Wiping the windows is everyone's first instinct but all it does is move moisture from your windows to a rag, without removing it from your car. So if you just entered the car, do not wipe the windows. Just put the defrost on high heat with high blower and allow it to work.
Pull in fresh air.
If you set your AC on recirculate, you just redistribute the moisture in your car. And your breathing and sweating create more moisture. By letting new, dry air circulate in, you effectively keep moisture out.