Many people buy timeshares with the intention of returning to the same resort year after year. But you're not locked into that, and when a thick exchange catalog arrives, showcasing castles in Scotland and ultra-luxury condos along the Portugal coast, it's hard to resist a trade.

To get the best exchanges, you need to own a timeshare that other people want to use. A family-friendly two-bedroom in Orlando during a high-profile holiday can give you fantastic trading power; a one-bedroom timeshare in a dingy resort during its shoulder season will be hard to swap. If you're a person who's buying for the exchange value, "buy the biggest and the best you can, but also at the highest season," The Holiday Group's Skinner says.

Two main companies run the timeshare exchange game: RCI (www.rci.com) and Interval International (www.intervalworld.com). These companies offer thousands of properties to exchange between, for a price—annual membership dues and a fee for each week exchanged (costs that are not covered by your yearly timeshare maintenance fee). Most resorts are affiliated with one of these companies, and a free year's membership often comes as an incentive with a timeshare purchase.

Along with RCI and Interval International, several independent companies work to set up timeshare exchanges. Many of these offer free membership and lower exchange rates with a lot of personal attention. "What we're all about is service," says Donita Cilch, who runs Dial-an Exchange, where customers can either request a property before they exchange or save up weeks in advance to plan a special trip.

Successful exchanges come from planning ahead and being flexible about when and where you travel. "The sooner you put in your request, the better chance you have of getting what you want," says Chris Boesch, PR Director for Interval International.