This is especially important if you're not sure when you'll be returning the car. When in doubt, reserve it longer than you need. You'll never be charged extra for returning it early, but returning it late can be costly. It's also important to reserve your car at least a week in advance to get the best deal. Rates can vary greatly at busy locations.
Popular destinations tend to have competitive rates that change frequently.
Keep in mind that rental car companies may vary their rates from city to city due to market changes. BreezeNet's Guide to Airport Rental Cars is an excellent guide for popular airport destinations, with special sections for renting cars in California or Florida, as well as a frequently asked questions page and special online discounts and coupons. BreezeNet also offers comprehensive information on rental coverages and gas options. However, these plans often end up costing more. You're better off declining these offers and making sure you fill up the car right before you return it.