Generally speaking, adopt a reasonable approach: do not operate electrical systems unnecessarily. Turn off the functions used to heat the seats, demist the rear window or ventilate the cabin, or set them on as low a level as possible for your requirements.
Air conditioning pumps energy from the engine. When it is hot outside, fuel consumption can rise by as much as 25% if you keep the cabin at a low temperature. Use air conditioning wisely and, if you have an automatic system, avoid selecting a temperature that is too cold or more than 4 or 5° degrees lower than the outside temperature. As well as saving fuel, you will avoid ""heat shock"" when you get into or out of the car.
If your car has been parked in the sun for a long period, start off at a low speed with the air conditioning off and all the windows open to let the heat out. The air conditioning can then take over to maintain the cabin at a constant temperature when the windows are closed.