I came across this thread and decided to share a tip I was given. If you want to drive one of the 'more expensive' or nicer rental vehicles (Premium,Luxury,Sports cars, large/nice SUV's, vans), make your reservation for the least expensive vehicle you can find and when you get to the rental counter tell them what you really want to drive. You will get a MUCH BETTER deal upgrading into your nicer/larger vehicles than trying to reserve them. I've done this countless times, and it's by far better than any discount code will give you to reserve that class of vehicle.
Example: I reserved a subcompact several months ago for $19/day and told the Avis rep that I wanted to know if he could give me a nice deal on one of their Camaro Convertibles. He got it to me for $75/day, as opposed to the $165/day that was the cheapest quote I could get to try and reserve one.
A friend of mine is a musician and rents a vehicle for a week at a time to haul his equipment. He does the same thing and gets the minivan that he wants for under $40/day as opposed to $70-$100/day, saving him anywhere from $200-$400 on every rental after all of the taxes, etc.
I've done the same thing when I've needed a larger vehicle (Tahoe, Suburban, Minivan) for the room. I've also told numerous co-workers and friends/family members who have done the same. The biggest key to really getting what you want at a very nice deal this way is to BE NICE TO THE RENTAL AGENT!!! As the old saying goes, 'You'll catch more flies with honey than vinegar'. ^
I should add that I only rent through Avis and so does the individual that gave me this advice, but I would think that all of the rental companies would work the same way...