I usually rent from National (wonder why I'm the National Forum moderator on Flyertalk). There is a list of codes that discount the rate (sometimes significantly) - my advice is to stay away from ones that include LDW (Loss Damage Waiver) since most of those are for corporate customers and if you have an accident, you may be faced with a situation that leaves you totally uncovered (by your personal auto insurance & credit card secondary coverage). Codes that I routinely use are 5007125 (generic Emerald Club code), 5125601 (AMEX), 5435776 (Travel Alternatives Group), and 8572130 (Air France / KLM Flying Blue - this one includes LDW but is safe because its for members of the frequent flyer program - just sign up to be "safe"). Combine these "discount" codes with a coupon from the list (Entertainment Book coupons are safe) or from National's own website - you're likely to get 1 day free on a 3-day weekend rental - or $20-30 off a week long rental. Furthermore, if you sign up for Emerald Club for free (see the National Forum on Flyertalk), you never have to stand in line at the airport - simply go and pick the car that you prefer from the Emerald Aisle (often upgraded cars are there) and drive to the checkout gate. Emerald Club members also earn 1 free day after every 7 rentals - save these for use on expensive rentals (one ways - or single day mid-week rentals). If you rent at least 12x a year, you get Executive status which entitles you to the Executive Selection (I rarely drive anything less than a Premium) and earn 1 free day after 6 rentals.

Feel free to ask questions if you want :)