Call me cheap, but I kinda like to travel smart. A "free" upgrade from a fuel-efficient compact to a midsize SUV won't be so "free" if the SUV drinks twice as much gas and has a gas tank 2x as large. That's what went through my mind at the Alamo rental garage. That's like getting a room upgrade at a hotel and then spending all the money you saved (and then some) on room service.

Make sure the car rental company you book with is located on-site at the airport. Yea, there's a ridiculous airport concession charge, but the convenience and time you save is worth it. I hate waiting 15min for a shuttle to take me somewhere else, and then having to take the shuttle back on the departure date. Because when you're on vacation, 15 min seems like 3 weeks.