How to rent a car

1) Get a driver's license. These are remarkably useful things to have, especially in North America.

2) Get a credit card that insures rental cars against collision and loss. Such cards often charge an annual fee, but will pay for themselves if you rent as rarely as once a year - buying that insurance from the rentacar company often costs ~$15 per rental day. Make sure that card is paid up, as the insurance may lapse if you're past due, and bear in mind you have to use it to rent the car.

3) Book online, in advance, preferably with at least one Saturday-night stay. I generally use Expedia to comparison-shop the various major chains, then go to the cheapest chain's corporate site and book a car there. I rarely wind up paying more than $25/day.

4) You don't need to provide a credit card number to rent a car, so feel free to book more than one, just in case.

5) Save money when you book. The bewildering rentacar business model includes all manner of "promotion codes", "discount codes", etc. Taking 30 seconds to find such a code online at places like RedFlagDeals or MouseSavers (of all places) can save you 15-20% or even more. Also, check the company's "Deals" page for special offers. Also, note that renting for a week can be cheaper than renting for 4 days.

5) Watch out for gotchas. Your mileage may not be unlimited. You may not be allowed to drive out of province/state/country, or if you do, the terms and conditions may change dramatically. (For example, Thrifty and Dollar in Quebec forbid you from leaving Quebec; Enterprise changes from "unlimited mileage" to "expensive, limited mileage" if you cross the border into the USA; Avis does the same trick on weekends.) Don't lie; many rentacars have onboard GPS trackers.

6) If renting in Europe, or actually anywhere outside of North America, either be comfortable driving a stick, or make it very very clear that you want an automatic.

7) If you are going to a faraway place, you may need an International Driver's License, which your home country automobile association can provide. However, it's an annoyance and only lasts one year. In practice, rich countries generally just accept each other's licensing systems, and poor countries are more willing to rent without such paperwork, so I've only ever needed one of these in South Africa, which only accepts foreign English-language driver's licenses. (Quebec's licenses are in French. This has never been a problem in the USA, but can be elsewhere. They also don't obviously indicate date of birth, which is annoying.)

8) Look for smaller, independent car rental places if you don't have a credit card, or if you're under 25, or if you want to rent a luxury car, or if you want to rent long-term. In particular, I recommend Super Cheap Car Rental if you need a car in California, esp. long-term, and you're not automotively vain: they provide well-used but well-maintained vehicles for reasonable monthly fees that include all insurance. Also, they're nice people.

9) In the USA you're often asked if you want "liability insurance" for ~$20/day (which is sometimes more than the car itself!), and it's incredibly hard to get a straight answer on whether you actually need this or not. In Canada the cultural context is such that the notion of going to and/or doing anything in the USA without copious insurance, lest you break a leg or get sued or something equally awful, is perceived with great trepidation. My impression is that rental car companies are generally required to provide the legal minimum of liability insurance, but I dunno whether this covers you out-of-state. In general I turn it down and then drive nervously.

10) Sometimes one-way rentals are very cheap; I once got a one-way rental from Phoenix to L.A. for $9.99/day and no extra one-way charge, presumably because Avis had a glut. Sometimes they're very not. Sometimes it's free to return a car to a different location in the same city where you rented it; sometimes it's not. Check in advance.

11) Check the car for damage before you drive it away, lest you get charged for it later. Although the one time this happened I sent them a scorching email and they immediately dropped their claim.

12) If your license has been suspended, don't rent from Thrifty, as they will actually check. (This is how I found out my U.S. license was suspended some years ago. I had been renting cars regularly from other companies for months.)

13) You will have an noticeably easier time at the U.S./Canada border if you're driving a rental car; the guards presumably figure "well, they might be dodging the cops, smuggling drugs, and planning to work here illegally, but hey, no problem, they'll never escape the wrath of Hertz!"

14) Don't return the car late. You get a "grace period" of maybe an hour, or even less; after that they start charging you usurious fees. Also, try to return it full, or they'll charge you amazing amounts for the gas you didn't put in the tank, and then rent the same car, unfilled, to the next customer and tell them to bring it back with the needle where it began. I think rental companies actually make most of their profits from surcharges and insurance.

15) If you leave something in the car, they're actually really good about getting it back to you; Avis once mailed me (prescription) sunglasses I had forgotten from California to Canada, on their dime.

16) If you drive a rental car to Burning Man, try to wash it thoroughly and then bring it back at a 24-hour automated-return location, because the employees will be rather upset with you, and not without reason (the engine will be coated with playa dust even if you never popped the hood.)