You can try to see if it applies online--then show your ID if they want it
You can also register for e-z car (we use them in ATL ALOT!) and they have extra 5% for airline people)
I would guess you can also try interlinetravel.com free membership with links to airline discounts--Budget has always been a good choice--sometimes Dollar has good rates. Fla car rentals are some of the cheapest in the States. Since I avoid Miami like the plague, I can't tell you current price deals and I personally wouldn't drive a car there anymore, but that's just ME...