It is usually cheaper to book your car hire online in advance of your date of travel than if you just arrive at a car hire location and hire a car.
However it is often cheaper to go to the car hire brands own websites, as they sometimes have special offers and deals when booking directly with them.
Before booking, try Googling car hire brands followed by combinations of "voucher", "code", "discount", "offer", etc. For example "Hertz voucher code", "Avis offer", "National Car Rental discount code", and so on. Many of the major car hire brands run promotions from time to time which could save you money on your booking.
Car hire companies will usually give you two fuel options. One is that you bring the car back empty, the other is that you bring the car back full.
They will perhaps say that bringing it back empty is more convenient for you, as you don't have to worry about finding a petrol station.
However bringing the car back full of fuel is always cheaper, as they charge considerably more than the actual cost of petrol or diesel for this service.
You will inevitably be asked if you want to take out insurance. When you hire a car in the UK you are automatically covered by the company's basic insurance policy, but this usually includes a high excess charge.
So, for example, if you had an accident that resulted in damage to the car you would have to contribute a large sum (the excess) to the repairs. This excess can be several hundred pounds.
The car hire companies have extra insurance policies which reduce the amount of the excess, for example Hertz's SuperCover and Avis' Complete Protection Package. But these can cost around £20 a day ($30 / €25), so on a long rental it can become quite expensive.
A much better option is to arrange separate car hire insurance in advance, and there are several companies which specialise in this including:
Also, check with your credit card provider, as some credit cards include car hire insurance.
If you have your own GPS unit, take it with you. Car hire companies charge per day for navigation systems, and again, the price soon adds up.
Alternatively, do it the old-fashiond way and by a cheap road atlas!
It is worth joining these schemes and collecting points towards further rentals, and some also offer free rental days or upgrades.
Often the best way to find out about discounts, voucher codes, special offers and promotions is via the car hire brands themselves.
Sign up to their mailing lists and newsletters, and if they have a Facebook presence then "Like" them to ensure your best chance of getting a good deal.
Many of the car hire companies will only accept a booking made with a credit card, so a debit card or cash isn't always acceptable. The reason for this is that a temporary deposit is charged to your credit card for the duration of your rental.
Even if you have booked your car hire online in advance, a credit card is often still required to secure the deposit. Without a credit card you wouldn't be able to drive the car away, so make sure you have a credit card with you!