from consumer reports magazine:
Resist the pitch for insurance. The car-rental agent will probably attempt to persuade you to buy insurance to cover any damage to your rented car. But your own automobile insurance policy might already provide protection. Most private policies cover rented cars as long as the person who signs the rental agreement is behind the wheel. If you have any doubts, check your policy or call your agent. Some credit-card companies also pay for rental-car coverage as a benefit for premium cardholders. Just make sure you use that card when you book your rental.