Some memberships will give you an actual discount when renting a car. The two best known memberships that give discounts on car rentals are: Sam's Club and Triple A. At many car rental places, these are good for a 10-15% discount, though they can not be combined. It is one or the other. Always ask a car rental place ahead of time if they give a discount for these memberships. With how expensive renting cars can be, you want every discount you can get.
Having Sam's Club or AAA membership will entitle you to some discounts or help, and it is always good to find out what exactly you qualify for ahead of time. Not only might AAA give you discounts on renting a car, and the road side assistance that they are famous for, but there is also a chance that the particular policy or membership you have with them might even carry a type of car rental insurance. Since this costs you more money than almost anything else, this would be a great deal. If you have an AAA credit card there is an excellent chance that they will automatically cover you with car rental insurance, as long as you charge the rental to that credit card.
Sam's Club and AAA are built off customer service, so don't be afraid to use the services they offer! Both have websites that are easy to find online, and offer extensive lists and information on how they can help, what services are provided, and who you can call or where you can go to meet with someone in person. These are great resources, as they will be able to tell you about car rentals in far more detail than what you might be able to find yourself.
AAA representatives can look up the city taxes and fees on your rental, which places have restrictions that will cost you more on vacation, and which don't. If you are trying to choose between Chicago, Boston, and New York for vacation, does it make a difference to know that New York and Boston are the most expensive cities in the United States to rent a car, each costing over $100 a day because of fees and taxes? It makes that third city much more appealing, doesn't it?
If you don't belong to Sam's Club or AAA, consider joining. Especially if you travel a lot, the benefits quickly outweigh initial expenses, and it is nice to know that even in a foreign country where you don't know anyone that if something goes wrong, help is only a call away.