If you want your car to drive like a badass, but use fuel like grandma, then only keep items in the car that you actually need. There is no need to store your entire wardrobe or something you might need one day, right there in your car. (Except re-usable shopping bags of course!) Take items and rubbish out of your car regularly and ensure that your car stays clean and organised. Just like you, your car needs to watch how much extra baggage it is carrying around.Put simply, the amount of of extra weight in your car increases your fuel consumption and slows you down. And you should know that roof racks add aerodynamic drag which also decrease fuel efficiency, so take em off when not in use. Remove any attachments from the exterior of the car that will add resistance to the car whilst driving.
For every 50 kilograms additional weight your car is carrying, your fuel bill increases by about 2%. More fuel is required to move a car with more weight. While you can't reduce the weight of the itself, you can minimise the amount of additional weight.
If you are going on a holiday and you have a lot of gear in your car, you will generally need more air in your tyres. This will ensure that your tyres are accommodating for the extra weight and that your car handles as well as it should. For more information refer to Why Your Car's Tyre Pressure is Important.