The best way to find a great deal when renting a car is to check competitive rates, since rates are constantly changing due to supply and demand.
Clubwww1 shows several rental agencies in most major cities, so you can shop around. That being said, book as far in advance as you can (at least a week) because car rental companies book up quickly, especially during peak travel periods.
If you have to change your plans, then act accordingly so you do not incur extra charges. Some companies have cancellation policies where charges are.