Try to find a deal where you can cancel for free up to 24-48 hours before picking up the car. You can get a cheap deal by booking early but can search again the week before going for last-minute bargains.
Petrol
Some firms have a "full-to-empty policy - you pay for the full tank (usually at an inflated price) when you pick up the car and can return it empty. It sounds convenient but adds extra cost, particularly if you are not going to drive far and won't use the tank you paid for.
Drivers
Some firms add a second driver for free, others charge by the day, which can add up. Drivers aged 21-25 tend to pay a premium.
Satnavs
Don't opt for one - the car may have it built in. Use your smartphone, particularly if you can download an app like Here, which works offline so you won't incur roaming charges.
Add-ons
Only buy the basic package. Firms make their profits by selling add-ons. In particular, don't buy "super collision damage waiver" (CDW) insurance - arrange your own cover. Avoid extra personal possessions cover as it is a waste of money.
Check the car
When you collect the car, mark every blemish or scratch, however minor, on the agreement. Pay particular attention to the wheels, especially alloy ones. Check the spare wheel is inflated and undamaged, the tank is full if it's supposed to be and record the mileage. Once you've marked all the damage on the form, get the rental desk to sign it. Don't leave without getting it signed as there is nothing to stop the firm arguing you caused the damage. Take a video or pictures as extra proof. When you return the car take photos of each panel of the car, the wheels and the odometer. Hand the keys back to the staff and ask them to inspect the car. Ideally, get them to sign to confirm there is no new damage.