Try one of these... depending on how long you are going to have the car will effect what type of discount code / coupon you can use as well as what type of car you want/need.
The below may or may not work, I did not actually test them.
20% off standard daily rate
If you know someone with an Entertainment book, you could ask them if they are not going to use their rental car coupons... Enterprise sometimes has coupons in there.
If you go to the Entrprise website, they are currently running 50% off on weekend rentals.
Just an FYI, a frequent flyer code doesn't really have anything to do with Enterprise or earning a discount, since Enterprise is not partnered with any Airlines. Also, the frequent flyer code would only get your frequent flyer miles, not a discount.
If you need further explanation let me know.
I hope this answer helps you out.
Edited to add: Do you HAVE to rent from Enterprise?
You might be able to find a cheaper rate or rate that is comparable to Enterprise from one of the other car agencies.
If you don't HAVE to rent from Enterprise send me another email and I can send you some codes I have for National that have gotten me some pretty good rates for Intermediate to Full-Size cars. These are no "coupon" codes, you just enter them when you make the res online, nothing to bring to the pickup site.
Book now with Enterprise Rent-A-Car!.This page contains a list of all current Enterprise Rent-A-Car coupon codes that have recently been submitted, tweeted, or voted working by the community..About: "Reserve a car rental from Enterprise Rent-A-Car at low rates.