Try some of the codes listed on mousesavers. I ended up getting a pretty good deal (minivan for $199/week for early Dec.) using the code "KISS2" from mousesavers, with Dollar. I booked that just to lock in the deal, but since you can cancel with no penalty, I am now checking periodically to see if I can get a better deal naming my own price on Priceline. (Supposedly, some people have gotten a minivan for as low as $19 a day on Priceline!) You can also periodically check to see if you can get a better deal using coupon codes, etc. as you get closer to your travel dates, but at least you'll already have a "fallback" reservation if the rates go up rather than down from what you booked at. (This is the nice thing about car rental reservations as opposed to flights -- there's no downside to booking early because you can always cancel if you find a better deal -- except with priceline, which is non-refundable.)