Present your auto rental voucher. Multiple bookings with the same supplier (perhaps you made a reservation online which was never cancelled) are confusing to rental car suppliers. If a written booking confirmation is not presented, the rental agent may locate the wrong reservation and a different — higher — rate could be charged.
Call the toll-free-from-Europe help line. Provided you book with Gemut.com, most issues that arise at the rental counter can be solved in minutes via a toll-free-from-Europe, 24/7 customer service help line (the number is under Terms & Conditions in your voucher). Let's say you booked an automatic transmission car but one is not available when you arrive. Call the number. Promised an upgrade, but the rental agent offers a Lupo? Call the number. It's your safety net.
Decline insurance. Most credit cards issued by North American banks offer CDW/Theft protection for auto rental in most of Europe. There are exceptions like Italy and Ireland, where the customer must purchase full-coverage. In all other countries, I recommend clients decline CDW/Theft in favor of the coverage offered by the credit card. Call your credit card issuer for details. Be sure the contract you ultimately sign does not include this optional insurance. If the counter agent says you have to purchase insurance, call the toll-free number mentioned above.
Decline pre-paid fuel. This is a bad deal. The offer works this way: you pay for the first tank and return the car empty. Not as easy as it sounds and who wants to be driving around on fumes, especially when headed for a European airport to catch a flight home. Any fuel left in the tank at the end of the rental is yours, but you won't get a refund.
Ask for instruction on vehicle operation. It once took me a full five minutes just to figure out how the windshield wipers on a BMW. Radios, too, are often not user-friendly and may have unfamiliar features. If you get a car with a GPS be sure it's set on English language mode. Finally, be sure you know whether you have a gas or diesel engine. Figure on a charge of about $400—and major inconvenience— if you fill the tank with the wrong fuel.
Inspect the car. If the car is dirty inside or out, refuse it. Check for obvious and not-so-obvious damage (even small scratches). Make sure any damage is noted in writing. If you have a digital camera, it's a good idea to take pictures of the car from all sides and again when you return the car. Visually inspect tires for wear and inflation level. When in doubt, call the toll-free help line.