Join If: You travel a lot or you have frequent (or long) stays at the same hotel chain. For frequent-flier programs, it usually takes about 25,000 milesto earn a free domestic round-trip flight in coach. For hotel-loyalty plans, you can generally count on spending at least $1,000 before you get a free night's stay. Both programs are free.

Options

Most major airlines and most hotel chains offer these plans. Go to www.webflyer.com to find information and enrollment forms for 37 frequent-flier programs and 15 hotel-loyalty plans. You can also find information there on airlines' partnerships ("alliances," in industry-speak), which are groups of companies that honor one another's frequent-flier programs. Oneworld Alliance, for example, includes more than eight major airlines, such as American Airlines and British Airways, and their affiliates.

To maximize the value of your membership, consider getting a credit card that earns mileage rewards on your airline. (You should be able to find card information on the airline's website.) Many hotel-loyalty plans also let you earn either hotel points or airline miles (you need to choose at the time of check-in), but the reverse is rarely true.