If you're planning to rent a car anytime soon -- particularly an economy model, consider meditating -- or whatever else settles you down -- before you get to the rental counter.

Maybe your plane has been delayed, canceled or whatever complication the airline has served up (along with the 3- ounce cup of soda and nine peanuts). Maybe you're carrying 20 pounds more than usual because you don't want to pay to check through a second bag. The agent at the rental counter probably isn't having a great day either.

Be prepared for the likelihood that the car you reserved, after carefully comparison shopping the competition, may not be available. If you like free upgrades, you're likely in luck. Reserved a compact and would like a Cadillac or an SUV? If the agent doesn't offer, ask. The rental car companies can't turn their fleets over fast enough to keep up with soaring reservations for gas thrifty vehicles. One solution to their own rising costs is to keep the fleets smaller.
This is more of a challenge if you're renting at a satellite location, where fewer cars are available anyway.

If the car you reserved is actually there, don't be surprised if it hasn't been cleaned yet. You may have to choose between waiting and taking it as it is. The rental agent probably has gas mileage information for the models available. If your own car is a Honda Civic, it might be a luxury to drive, well, a luxury car for a few days.
It may cost a little more in gas but this is one situation in which the better deal is the bigger car.