I buy and Entertainment book. This years' right now is probably 20-25, but even at 35-40 (depends on where you are), you'll save more just using the entertainment code (about 25-40 percent), and you can use coupons on top of that. If you use an online coupon, you can use it a million times if you want, they don't limit them (they don't for me, at least). my last rental, with no discount would've been about 55/day for 4 days (220+airport fees+taxes I think was about 290). With the disounts, it was 145, prepaid, with taxes and fees. I paid 20 for the book, and saved 145. I always look at before/after coupon, because I try to figure out which coupon gives me the best deal on a rental. I personally prefer avis, but they have coupons for Alamo, National, Hertz, Thrifty, Enterprise, etc., mostly the same kinds, so just pick one and go.
*Edit-I failed to mention this was for the car just under luxury, not the cheapest. I'm sure the cheapest would be cheaper than that, but usually the difference in price between each level may only be 10 bucks over the whole rental, depending on the car, and that's my preference (2nd highest level). and even if you opt for full size (3rd highest), it's typically a 35-40mpg car anyway, so you get a nicer car, that gets great mpg, for a few more quarters a day.