Whenever I need a rental car, I pay no attention to the different vehicles offered on a rental Web site and book the cheapest car available. It's not that I love econoboxes but that I know I'm likely to get upgraded to a better car for little or no extra cost.

Rental-car companies overbook, just like airlines, and the economy cars are usually the first to go. By booking the most popular model, you're likely to get a better car without even asking for it.

What day you're walking onto the lot can also make a difference: In big cities, companies that rent cars to business travelers during the week will often hand you the keys to a nicer car at no charge if you show up in the middle of the day or early in the week - when most business travelers have cars out.

And the reverse is true at resort destinations, which do the bulk of their business on weekends. The best time to score a free upgrade there is on Saturday afternoon, when most of the lower-end cars have been rented out.