Reduce the load

Only carry what is essential in your car, clean out any junk or necessary items otherwise you are paying for the fuel to cart these from place to place.

Reduce the speed

I believe wind resistance increases with speed (Quora, help me here) so avoid excessive speed where you can.

Drive smoothly

As you break you are heating your brakes. This heat energy is a transformed version of the money you paid to fill your car with fuel. Drive smoothly to avoid the need for heavy braking and so throwing this fuel money away.

Reduce the wind resistance

Close your windows and remove items that would increase the drag on your car. Do you really need that flag on your car aerial?