How to get car rental upgrades cheap or free

First, there are several initial factors that can help you score a nice ride:

• If you are arriving on Sunday night, many cars are coming back into the lot.
• If you are arriving on Friday night or Monday morning, rentals are on short supply. "What, you don't have the car I reserved? Upgrade me for free."
• If you come to Vegas when lots of conventions are happening, you are in luck. It's a sure sign car rental places will be short on cars. As long as you have a reservation, they'll try to accomodate you.
• Sign up for the rental car rental membership (most are free.) It helps to have a member number when trying to negotiate.

When at the counter, always ask if there's any weekly specials. When weather's bad, convertibles are the first on specials list. Mazda MX roadster? Can be easily had for same price as a standard class. I've had everything from Nissan 350Z to Infiniti G35 on specials (at about $50/day.)

Ask if they are willing to premium or fun cars that are less likely to be rented (like Jags.)

If you arrive when they are very busy at the counter or when they are short on cars, take your time negotiating. They are looking to reduce the queue so they will negotiate if you are taking too much time.

Final but effective tip: Call in ahead before arriving at Hertz. Find out the name of the manager and see if he's willing to give you a special. "I want to chauffer around a friend getting married", "I'm coming in for a convention and hoping to find a good deal on a rental.". Even if you don't get a verbal discount offer, knowing the manager's name can be valuable.

When you are at the counter, drop the manager's name to the counterperson and say you were offered a discounted price of $xx for a car.