Fill out the car condition sheet before you leave the rental agency, and mark every little scratch, ding, or dent no matter how minor it may seem. If possible, take a video or pictures of the damage as well. Be sure to thoroughly inspect the interior for blemishes, frays, or signs of wear. The 20 minutes you'll spend doing this could save you a lot of money in the end. If the car appears blemish-free, protect yourself by taking several full body shots from each side, front and back. Don't forget to get a picture of the windshield and even of the undercarriage. You could even take a picture of the rental agent, with his permission.
Pay with a major credit card. Most credit card companies already provide collision protection. Rental car companies provide this same coverage, but it can run as high as $25 a day. Your own personal insurance may not cover you, but most credit cards, including Visa, American Express, and MasterCard, provide collision protection. Check with your credit card company before renting the car to be sure you're covered.
Take the rental car agency's add-on insurance if you're not covered by your primary insurance or a credit card. It's in your best interest to do this, in spite of the hefty price tag. It may save you from a hefty damage bill later. Some countries require you to take the rental car agency's insurance. Be sure to list all potential drivers on the contract.
Test all the major systems in the car before leaving the lot. This includes the air conditioning, radio, windshield wipers, lights, and transmission. Report anything that's not functioning immediately to the rental car agency and refuse to accept the vehicle until it's been clearly noted.
Present your pictures, video, and thorough car condition report if you should still receive a bill in the mail after all your precautionary steps. Escalate to the next higher level of management, if necessary, to present your case. The best way to argue against damages is to show the evidence that you gathered up front before signing for the vehicle.
