A cheaper rental will help offset the cost of gas. Check out the booking site Autoslash.com, which searches the Web for discount coupons on car rentals and applies them after you book. It then continually checks for lower rates and coupons up until your trip date, and automatically applies any discounts it finds. Willing to gamble? Consider websites like Priceline.com and Hotwire.com, which offer deep discounts to travelers willing to be locked into a preset price before learning what company they'll be renting from.