When booking a car rental two common membership discounts you may qualify for are either AAA or AARP.
While Hertz is AAA's official car rental partner (and offers a number of special deals and extra benefits to AAA members), Budget also provides a AAA discount.

As for AARP members, Avis, Enterprise, Budget and Hertz all offer discounts. If you quality for both AAA and AARP, then you should compare them but at Hertz usually AAA will trump other discounts.
Here are the codes you need in order to check the rates on-line or with a call center:
| Company - | AAA discount - | AARP discount |
|---|---|---|
| Hertz | CDP # 005 | CDP # 16131 |
| Avis | n/a | AWD # A359800 |
| Budget | BCD # U089000 | BCD # Y508500 |
| Enterprise | n/a | Cust # RP72X7 |
Notes:
Each AAA club has its own CDP # but if you belong to AAA in northern California or Nevada (CSAA - California State Automobile Assn.) then your club code/CDP # is 005.
The AAA club codes won't change but the other codes that Avis, Budget and Enterprise employ can change over time.
Other car rental companies I checked (Dollar, Thrifty, and National) did not offer either AAA or AARP discounts. Alamo said they do offer a AARP discount but refused to disclose the pricing code so you'll need to call Alamo directly.
Other regional car rental companies may also offer AAA or AARP discounts.

Anyone know a good coupon from enterprise.com?
I checked Alamo and, for the car rental I was interested in, the price with AARP discount was MORE than the pre-pay price available to anyone without membership. Sadly Alamo did not offer a combination of AARP discount with pre-pay. Note that membership #12341234 lets you check prices although one would hope they would ask for your membership card at the time of rental.