Before hiring a car, shop around and check what is included in the contract. Some quotes may appear cheap but there may be hidden add ons, such as different insurance levels. Theft protection may be extra (it is mandatory in Italy). This is one of the times it is probably better to deal with one of the larger companies based in the UK. They are more likely to have roadside coverage, better models and quality of cars. It will be easier to complain if things go wrong too! Pre-book in the UK for ease of comparing prices and for what is likely to be a cheaper deal.
When you take the vehicle check tyres, fuel tank and bodywork. Take photographs with the date on them if possible. This is your evidence should there be any dispute. Take photos again when returning the vehicle if necessary.
If you have a complaint, write to the company. If not satisfied with their response then you can take further. For UK rentals, the British Vehicle Rental and Leasing Association (BVRLA), can help you with complaints about its members. It has a code of conduct which members must abide by and it will investigate if a member has breached this. It aims to resolve disputes within 30 days.
The European Car Rental Conciliation Service (ECRCS) is the dispute resolution scheme for a number of car rental companies throughout the EU. So check this list before hiring so you know that you can take a complaint to an independent body if you have a complaint. This works in the same way as any other ADR scheme but for across the whole of Europe. In order to use ECRCS you must have booked with the company direct and not through a travel agent/broker.
There is also the European Consumer Centres Network, which has a free service to help resolve disputes and help you get your money back.
I wrote an article for the Financial Times which provides lots more advice. Avoid a holiday car nightmare
For all things complaining holidays see All you need to know about booking/complaining about holidays/flights
For more advice, tips and templates for complaining see GET THE BOOK! How To Complain: The ESSENTIAL Consumer Guide to Getting REFUNDS, Redress and RESULTS!