It may take a while and a combination of different procedures to finally figure how to balance the climate control of the interior of your car to prevent the windows from fogging up. The biggest problem is the different environmental conditions going on outside of your car and how to adjust the controls inside of the car to compensate for them.
Allow the car to warm up. This may sound peculiar, especially in the summer, but if you're running A/C, then the compressor needs to thoroughly cool the interior of the vehicle while the windows are adjusting the heat outside of the vehicle. Placing the flow of air to the windshield during this warm-up time will help speed it up. For colder weather, the same premise applies except with the heat on. Place the heat direction to the windshield and let it run to warm the windshield and windows up and adjust to the outside temperature.
Open the side windows just a crack. This may be annoying and create a slight uncomfortable breeze inside the car, especially on extremely cold days, but if your windows are fogging up on such days, it's because your body temperature (especially with multiple passengers) is exceeding the interior temperature of the vehicle and it's having a hard time adjusting to the exterior temperature. Keep the fan running on high and the directions aimed at the windshield (on defrost) even if its the side windows fogging up. Cracking the windows open slightly (pivot windows come in handy and can be cracked open to substitute) will allow a fresh air inlet to help prevent the fogging of windows.
Turn the temperature up to a warmer position. This may be hard in the warm summery days when the A/C is running, but it's very common that sudden temperature changes outside will wreak havoc with the temperature control inside your car. For example, it's hot and steamy, you have the A/C cranked and are driving along and suddenly hit a ferocious thunder storm. The outside temperature drops 10 to 20 degrees in an instant and your windows fog up almost immediately. Again, place the direction of the air to the windshield, but blow warm air onto the windows to adjust in the outside temperature drop. It still may feel humid in the car, but cracking open the side windows will help.
Purchase vent visors for your car. This allows you to crack your windows open (even just slightly) and prevent rain or snow from dripping inside the vehicle. If it get's too hot or cold, adjust the temperature control panel on the vehicle before deciding to close the side windows. Aiming the air direction on the windshield or on the windshield and floor position in the interior climate control positions will help out tremendously.
