Some things I have learned ...
Book your car for an afternoon pickup and return. Mornings, noon time and late evening pickups and returns will give a higher rental rate.
Set your return a bit earlier then your pickup time example: pickup at 3pm return at 2pm. Seems to give a better rate.
Once you book your car, check back a few hours or a day later and see how the price may have changed. I did this several times and so far I have saved nearly $200 for a 2 week booking.