I second using mousesavers.
I played with all the discount codes and coupons and booked my rental months ago with Dollar for our September trip for $300 for 8 day minivan.
I just kept watching and trying all the different companies and codes and ended up booking with Alamo a couple of weeks ago for $161 taxes incl for 8 days. That includes a free add'l driver.
When we went last year at Christmas, I starting looking to book a rental months ahead of time and it was really high. I got a notice from Hotwire about 2 weeks before our trip for $16.95/day for a minivan.
I would suggest just keep trying and hopefully some discounts will come up.