Build up loyalty by renting at the same location as much as possible. Agents who know you are more likely to do you a favor than those who don't.

Express an interest in the car you want. Knowing you're interested might be enough to prompt an agent to rent you a vehicle in the next class if there's an overly heavy demand for economy cars.

Pick up your rental as soon as the office opens in the morning. Most of the nighttime returns won't be ready yet, which is likely to lead to a shortage of economy cars.

Join frequent-renters programs at companies that have them.

Exchange airline frequent-flyer miles for car rentals or upgrades, if you have miles ready to expire. (Free flights are usually a more cost-effective use of miles, however.)