Starting to look for deals at least two months before travel will allow holidaymakers to access the best deals.
The consumer group also recommends using a Which? trusted car hire broker such as Zest, to help compare prices and access special deals.
Check the refill policy
Nearly all car rental firms have a policy on how much fuel customers need to have in the vehicle when they return it.
And Which? suggests that checking the fuel policy carefully will help customers save money.
The experts also say that a full-to-empty fuel policy, sometimes known as a pre-purchase, can also bump up prices.
Which? says it's touted as convenient, because customers won't have to find a petrol station before returning the car.
But they add people will end up paying through the nose for a full tank and sometimes even double the rate than at the pumps.
Find your own insurance
Car hire firms will often talk customers into panic-buying excess insurance when picking up their vehicle.
But Which? says they can find standalone insurance for less than £2 ($4) a day on comparison site Moneymaxim.
However, it warns that if customers purchase standalone insurance, the hire company will insist on an eye-watering deposit and they will still have to pay the excess if you have an accident, but you can claim this back from your insurer later.
Alternatively, if driving holidays are frequent, the experts say it is worth considering annual excess cover.
Choose off-airport hire
Picking up your vehicle from the airport is convenient, but Which? says car hire companies pay taxes to be there – increasing the price of rental.
The experts suggest collecting and dropping off cars at off-site offices, which are cheaper, and many offer a free shuttle service.
Avoid extras
Most holidaymakers know that adding extras at the checkout can send car hire costs soaring.
However, Which? says there are several things customers can do to keep extra costs down.
Instead of hiring a sat nav, the experts suggest downloading free apps such as Navmii or Here WeGo, which can access maps offline for more than a hundred countries.
When it comes to car seats, Which? advises that holidaymakers travelling with easyJet or British Airways can take one as luggage for free or alternatively parents can hire one at the airport for a fraction of the price.
Adding additional drivers can also cost more money, and the consumer group suggests checking with companies before signing up.
Meanwhile, holidaymakers who intend on spending a lot of time hitting the road should check that their car hire contract includes unlimited mileage.
