Investigate whether or not the credit card you use for the rental, or your auto insurance company, will cover the cost of any damage incurred while you are driving the rental for the day. Don't pay for the car rental agency's insurance coverage (often called collision damage waiver or loss damage waiver) if you know you have coverage fro your own insurance. Be aware that in some states this coverage is compulsory and built into the car rental cost. (Reference 4)
Bid for a rental car on a site such as Priceline.com or Hotwire.com. You might be able to get a daily rate at a considerable savings over renting directly from the car rental agency.
Refuse any expensive extras that can increase the rental bill. Global-positioning units is one such extra.
Read the fine print on your car rental contract. You need to know if you are responsible for filling up the car with gas before you return it that day, and what time you should have the car back to the agency so you won't incur additional fees.
