Get what you really need
Unless you're planning a Route 66 road trip, be sensible about what you realistically require from your rental car. If you don't need a 4x4, often more than double the price of the average family-sized vehicle, then don't bother. If a hatchback will do, then don't opt for a soft-top or a high-spec version, particularly if it's just for pottering to local towns or tourist attractions.
Do double check, however, whether aircon, sat nav are included if you're likely to need them. If you plan on sharing the driving be aware that some of the cheaper options specify one driver. To avoid a sizeable surcharge, name any additional drivers in advance as some firms offer an extra driver for free.
While car rental firms are required by law to ensure that all vehicles are insured, the standard package will not afford you the kind of cover you'd expect on your own vehicle.
Furthermore, the policy excess is often sky high, leaving you with a huge bill for the slightest scratch. While most firms offer an excess waiver, usually charged by the day, it may be cheaper to invest in a standalone additional insurance cover before you jet abroad. Economy Car Hire, though not the cheapest option overall, includes built-in excess insurance, while Money Maxim will compare insurance excess providers.
Also, make sure you specify and extras before you pick up the car, as you may face seriously inflated prices if you pay on the spot.
It used to be that picking up and returning a rental vehicle with a full tank was the norm. These days though, many companies will charge you up front for the cost of the fuel. While that means that you can bring the car back without a full tank, you'll generally find you are paying inflated petrol or diesel prices, and you'll also lose out if you return the car with fuel still in the tank.
Check for offers
If you have booked flights and hotels through a broker or travel firm, you may find they offer a discounted car hire rate so it is absolutely worth checking. In the US, flydrive holidays are particularly popular, and that's because you'll get the car hire included in your package holiday, which could save you a bundle.
And as always, the internet is a valuable resource when it comes to price checking, so try comparison sites Kayak, which also offers a useful app, Carrentals.co.uk and Travel Supermarket for the best deals.
Last but not least, try and pay by credit card. If the rental firm doesn't deliver on its promise, you'll at least be able to recoup your losses.