The Consumerist looked at 3 tactics employees use to get you to pay more. Like upgrading to a better car - Or getting you to pay for one type of car but you're really getting one at lesser value. Finally - trying to get you to add insurance.
First - upgrades
Always make sure you ask the price difference of a vehicle upgrade.Sometimes the employee won't mention the increase. You might not find out you're paying a few bucks more per day until you get the bill. Since you signed on the dotted line - there's not much you can do about it.
Second - paying more for less
Let's say you go pick up the car, but the one you reserved isn't there. Make sure the company is giving you a car in the same price range. The Consumerist said some companies try to give you a cheaper car but still make you pay the original price for the nicer car you requested.
Third - insurance
If you own a car, there's a good chance your insurance policy covers rentals. So before you rent a car - check your policy. Employees really try to get you to add the insurance. If they refuse to rent you a car if you don't buy insurance show them your insurance plan.