Just some tips for Avis which helped me on this last rental.
1. Check the rates on EVERY location in your area and make a reservation with the cheapest one. Then call the location you really want to rent from and ask them to match the rate since you would rather rent from them. It might help if the original rental had the car being rented from the cheaper location and being returned to the one you really want it from (usually you will still get that low rate even though the return is different). When I did that here, ORD had no problem matching the rate, even though it was over $200 cheaper than what they were currently offering. It could be because I'm PC. I don't know.
2. When you call to make a res, call PC reservations (800-651-2847) and ask the agent to call the location while you wait on hold to request the double-upgrade for you, or even more than that. I reserved a full size which bring me to a specialty with a double upgrade. The thing is they normally only upgrade to a premium even if that's only a single upgrade. They are told not to upgrade above that. I had the agent call the location and the manager there guaranteed me a luxury. Sweet.
3. If you have problems with a rate, it's usually best to wait and call customer service after the rental is over. Of course you may get screwed, but the few times I did it, they helped me out. Whether if it was when I got charged for some fuel, or if the total ended up more than I thought it would be, they helped me out.
4. It may be worth it to call the Chairman's Club desk (even if you are not CC) and ask them to help you with a special-needs reservation (like manually adjusting the rate). The agents there have much more leeway and I've never had them tell me "Sorry sir, you are a PC member, not a CC. I'll have to transfer you". In fact, I once got them to adjust a one-way rental for me from JAC airport to SLC to be the standard rate which normally comes with no miles (40 cents per mile), and change it to unlimited miles, which would have been $200 more expensive.
I hope these tips help some of you guys, but remember, don't abuse your privileges, and don't make a Chillul Hashem.