Name your own price
I'd recommend trying Priceline's name your own price offer. I did this the other day and managed to get a mid-sized car for 2 weeks from Hertz for $349, including taxes and fees. I entered in an offer of $15 per day, and on the next page, they gave the total price that it will come to with taxes and fees. Once I confirmed my offer and entered in my credit card info, it submitted it and then said, "congratulations, your offer has been accepted." If it is accepted, your card is immediately charged. Easy as pie. Word of caution though....I wish I had started with a lower bid- who knows if it would've been accepted or not. You'll never know unless you try, but as I recall, if the bid gets rejected, you can't try again for 24 hours, UNLESS you change the car type (like from mid-size to full-size). Anyhow, I had good luck this time, and last year when I rented a convertible for a week at $30 per day. Check it out & good luck to you!