When you get to the counter, ask if there are any upgrade specials available, they might have some good deals. Check for special deals with your memberships in associations or clubs. Memberships in Premium Clubs, loyalty or frequent flyer programs can offer discounts. Some credit cards have special discounts or upgrades with certain Rental Car Companies. Package discounts can be found when booking your rental car with certain airlines, hotels or attraction ticket packages.
Discounts are often available for government, military or corporate employees.
Some Rental Car Companies will even extend these discounts during leisure travel. Some special airport rates require you to present your arrival airline ticket. You may need to have arrived within 24 hours to get the special rate. If you are going to keep the car for more than five days, you can also check into the weekly rates. Generally the sixth and seventh day end up being free in the weekly rental.