I usually book a car rental when my plans for a trip firm up. Then check periodically afterwards, particularly when the "last minute" specials are announced on Tuesdays up to 2 weeks before the trip and then again the week of the trip. Be sure to check the generic Emerald Club code 5007125 along with any corporate codes you are "entitled" to use since the cost may go down particularly with the generic Emerald Club code. It's good to lock in a rate once your trip is firm since there is no cancellation penalty with car rentals then check if you can improve the rate.