Keep Your Options Open
Planning ahead isn't always the best way to save money; sometimes car rental places have last-minute deals when their car rentals aren't moving as fast as they want them to. After all, an unused car represents profit a rental place is not realizing. If you're willing to take a rental that isn't quite what you were looking for, you may be able to get a good last-minute deal from a car rental place.
Bid Your Own Cheapest Price
Using services like Priceline, you can set your own price; have it sent out to multiple car rental places, and wait to see who wants to rent you a car at your chosen pricing level. To do this successfully, make sure you check car rental prices in the area you're going to be driving in, and don't make your bid absurdly low compared to those prices. Your best bet is to not go below 20% of the lowest price you see in the area. If you don't get an offer, you can try again with a somewhat higher bid.
Watch for Coupons
In the Yellow Pages, in postal change-of-address kits, online, and in the paper you should be able to find coupons for discounts or upgrades on car rentals. If you use car rentals frequently, save these whenever you get them. Other places to find coupons include your AAA office, travel agencies, visitor information centers, chambers of commerce, and airport brochure displays.
Don't use the coupon without checking prices on other car rental companies, however. Often even with a coupon, prices on the same car will be lower at a competitor. You can also take your coupon to a competitor you prefer using and ask if they will honor it, even though it is for another car company. Sometimes they will.
Things to consider
Extras may cost more money. If you like having more insurance, or if you want to upgrade to a better or larger vehicle, you can expect to pay considerably more. When you're going from a low-end car to a somewhat larger car with insurance, the price can double.
Your car should at least be adequately large enough to carry everyone and everything you need to haul. If it's too small, you'll pay for it in discomfort and inconvenience. Finally, ask about emergency roadside assistance. Some car rental agencies will provide this at no cost to you.