The amount of water that can be suspended in air (as water vapour) depends on the temperature of the air -- the hotter the air is the more water it can hold. The warm air inside a car can hold a lot of water, but the air near the windshield is colder than the air in the rest of the car. This water must come out of the air right next to the windshield as a condensate, which causes the windshield to fog. There are several things you can do to keep the windshield from fogging up.
Open one or two windows slightly. This both cools and drys the inside air, and it is the fastest and simplest way to get rid of fog on the inside of the windshield. It only takes a small change to stabilise the interior and get rid of the condensation -- and keep it off the windshield. If you do a little experimenting, you probably can find a balance between the heater and the side windows slightly open that keeps the condensation off all the windows. Keeping water out of the car also reduces the water vapour in the interior air -- shake off umbrellas and kick as much snow and water as you can off your clothes and shoes before entering the car.
Turn on the defroster. This directs warm air on the windshield so condensation does not happen. Do not use the recirculate option as this keeps the same air inside the car and what you really want is to bring in exterior air. The exterior air is both colder and dryer than the interior air. In some cars, turning on the heater and air conditioner at the same time keeps the condensation down because the air conditioner takes moisture out of the air. If the heat control is turned up -- as it is when the heater is running -- the air conditioner will not be cooling, it will be acting as a dehumidifier.
Clean the inside of the windshield to prevent fogging before it happens. Fog has a harder time forming on a clean surface. Some cars -- especially new ones -- are constantly giving off fumes from seat covers and other plastic surfaces. This produces an invisible film on the interior glass surfaces, which make it easy for fog to condense. A few drops of white vinegar is the best solution for cleaning windows. In the toughest cases, glycerine (available in chemists and hobby shops) forms a thin protective surface on clean wind shields that prevents condensation.
When cleaning your windshield, do not forget to clean the inside of the back window. This also fogs up and can be hard to clear.
Do not use commercial window cleaners. Vinegar and water works better and is a lot cheaper. A stronger mixture (3 parts vinegar to 1 part water) can prevent the outside of your windshield from icing up overnight.