Many frequent-renter programs offer free upgrades to loyal customers.

Tip #4: Skip the Refuel Option
Unless you're in a huge hurry or not handy with a gas pump, the refuel options are usually a bad deal. Sometimes the per-gallon rate might be lower than you'd find at a gas station, but most rental companies charge you for an entire tank, even if the car is only half empty. Consider: A half-empty 18-gallon tank, refilled at a station for $3 a gallon, would cost $27. A fuel purchase option, even at a less expensive $2.50 a gallon, would still ultimately cost more, totaling $45. Look for a gas station near the airport or rental outlet at the beginning of your trip so you can fill up before you return the car.

Tip #5: Be Cautious About Insurance
Many drivers assume they're covered by their credit cards. But the policy often relies on a driver's personal insurance company first, and the card picks up secondary expenses. Read the fine print, and, if in doubt, opt for the rental company's damage-and-accident insurance.

Tip #6: Don't Advertise Your Rental
Many states outlaw rental car agencies from putting their company stickers on the exterior of their cars. Why? Rental cars scream, "Hey, I'm a tourist traveling with lots of money, cameras, and other valuables." Be careful not to leave your rental agreement on the seat, and be mindful that some agencies have branded their GPS navigational systems with their corporate logos--making cars equipped with that option an easy target.