Just like your body, your car needs more fuel to move around more weight. So just as you wouldn’t wear a heavy rucksack unless you had to, don’t cart stuff around in your car boot unless you really need to. Ironically, the heavier the item the less likely you are to bother taking it out of the boot and the greater the effect this will have on your fuel consumption.
Also, you may not be aware that when you drive a car that’s been parked for a few hours the engine is cold and it therefore uses much more fuel for the first five miles or so? To conserve your petrol or diesel avoid popping out here and there, try to combine your daily errands into one trip.