There are plenty of ways to get a free upgrade or other bonuses/discounts in the car rental industry. You can google for the promo code, monitor rental company promotions page, subscribe to their mail list, etc. One option (and it can be used with all the offer coupons you find) is to register with one of the credit cards offering some sort of a status for car rental members. Here is the list of these credit cards, check if you have any in your wallet.
- National Emerald Club Executive
- World Elite Mastercard
- Visa Infinite
- Any American Express card
- Avis Preferred Plus
- World Elite Mastercard
- Visa Infinite
- Sixt Platinum
- World Elite Mastercard
- Hertz Five Star
- Visa Infinite (cardholders based in Europe)
- Hertz Presidents Circle
- Chase United MileagePlus Club Card
- Chase United MileagePlus Presidential Plus‎ Card (grandfathered)