Before opting for extra equipment, compare the cost of renting it from the car company to that of buying it outright.
If you own a portable GPS navigator for your car, consider downloading a map of Europe into it. Avis's cost for renting a GPS device was $18 per day ($144 for an eight-day rental). The cost of downloading data for all of Europe into my Garmin GPS navigator—which means I can use it for future trips as well—was $160. What I did opt to rent from Avis was snow chains for $99, since we were headed to the ski slopes of Spain's Sierra Nevada. Then I discovered at a roadside supermarket that I could have bought chains for a mere $31.

If you're given a diesel car, find a way to guarantee that you don't fill it with gas by mistake.
Travelers to Europe are increasingly being given diesel cars—a good thing because they are ecologically more sound and save you money on fuel. Unfortunately, according to the British Automobile Association, each year 150,000 renters in Europe put gasoline into a diesel fuel tank. So don't leave the rental agency without learning the local word for diesel. When Avis in Madrid handed me a diesel car, I wrote the Spanish word for diesel—gasoleo—on a piece of masking tape and stuck it on the gas-tank cap.

Return the car when the rental office is open and manned so that you can get a written record of its condition.
I had purposely chosen an Avis branch that's open at night because I wanted to make sure someone would be there to inspect the vehicle at drop-off. Why? One of the most common car-rental complaints I hear is from readers who returned a vehicle in good shape and then, weeks later, got hit with a charge for damages they didn't cause. So take photos of the car—at pick-up and at drop-off—to document its condition. (If you see any scratch or flaw at pickup, don't merely photograph it; have the rental agent document it in writing.) If you are returning a car in good condition, ask the rental agent to write on your contract that the car was brought back damage-free. When I asked the Avis agent to do so, he printed out a copy of my final bill, assuring me that this guaranteed my credit card would not be charged for damages.

When you return the car, do not leave without a final itemized bill that you agree with.
I could not decipher my rental contract (which was in Spanish) and, at drop-off at 8 p.m., had no time to query the Avis agent about the various charges on my bill: I needed to get my young sons, who were hungry and cranky after seven hours in the car, to our hotel. Back in the United States, I contacted Avis for an itemized statement I could comprehend. For reasons that remain a mystery, I was never billed for the snow chains. I was billed for two additional drivers—which is odd because neither my pre-schooler nor my kindergartner has a driver's license yet. I was also charged a "special cleaning fee" of $63—which was a complete surprise, but given the state the kids left the backseat in, I'm not about to quibble.