Why?
The air conditioning is one of the most fuel hungry accessories in your car, increasing fuel consumption by up to 20% when it is in use!  You can hear this when your turn on you're A/C while your car is idling - the engine starts to work harder. Your engine works harder to mechanically operate the compressor and electrically to run the blower motor. With a smaller vehicle the reduction in power is more significant and so is the effect on fuel economy.

Automatic climate control systems can cause the air conditioning to run, even in winter, to reduce moisture content. This increases the amount of time that your air conditioning is running and costing you that extra 20%.

How do I do this?
In the summer, rely on ventilation system and air vents to cool the interior rather than air conditioning. Roll down your windows when driving at speeds less than 60 km per hour. At highways speeds the increased drag of your open windows can offset your savings.

If you are using your air conditioning, use it intermittently (turn it off and on) and turn it off 5 minutes before you arrive at your destination to make use of the cool inside temperature that has already been achieved. At highway speeds, try using your car's flow through ventilation.  

If your vehicle has an automatic climate control system, either  manually shut off the air conditioning when not needed or select the "economy" mode.

When leaving your car parked, find a shady spot or use a windshield visor to reduce heat build up. Leave your windows open a small amount to allow some of the heat to escape from your car while it is parked.