Check into booking your vacation as a package. You might be able to save by booking your car along with a certain airline, hotel or attraction ticket package.
Book the smallest car that you will need and hope for a free upgrade. But, remember you may get stuck with that small car.
Check into other size class vehicles. They may have extra cars in a certain class and be offering discounts for that size class.
Use coupons, but read them carefully for exceptions. Most upgrade coupons are "based on availability", so if you have an upgrade coupon that you wish to use and also have a monetary discount coupon, bring them both. If they can’t honor the upgrade, they may still give you the monetary discount. Also, remember to book any coupons or discounts in your reservation.
Apply any special discount programs for which you are eligible.
When you get to the counter, ask if there are any upgrade specials available, they might have some good deals.