Virtually all major U.S. airlines have a frequent-flyer plan, and many foreign carriers are starting them. These programs allow you to earn free trips, upgrades (e.g., from Coach to First Class) or other awards based on how often you fly on that airline. In some programs you can earn credit by using specified hotels, rental car companies, credit cards, etc. It doesn't cost anything to join a program, and you can enroll in the programs of any number of different airlines. However, it may not be to your advantage to "put all your eggs in one basket" with one plan by accumulating a high mileage balance only to find out later that another carrier's program suits your needs better. Here are some things to look at when selecting a frequent-flyer program.

* Does the airline fly where you're likely to want to go?

* Are there tie-ins with other carriers, especially those with international routes? Is some of the airline's service provided by commuter-carrier "partners"? In both cases, can you earn credits and use awards on those other airlines?

* How many miles (or trips) are required for particular awards?

* Is there a minimum award per flight (e.g., you are only flying 200 miles but the airline always awards at least 500)?

* Is there a deadline for using accumulated miles?

* Carefully examine the number and length of any "blackout periods" during which awards cannot be used. On some carriers, the Thanksgiving blackout may last a week.

* If you are planning a big trip and are thinking about joining that airline's frequent-flyer program, enroll before you travel. Airlines usually won't credit mileage that was flown before you became a member.

After you join a program, there are other things that you should know:

* Airlines reserve the right to make changes to their programs, sometimes on short notice. The number of miles required for particular awards might be raised, requiring you to use your old mileage (i.e., your current balance) under the more restrictive new rules. The airline may cease service on a route that you were particularly interested in-or it may drop the city you live in! The carrier may eliminate attractive frequent-flyer tie-ins with particular airlines or hotel chains.

* Cashing in your mileage frequently will limit your losses in case the carrier changes the rules, merges, or goes out of business. (Some private companies sell insurance covering some of these eventualities.) Accumulating a larger mileage balance will entitle you to bigger awards, however.

* Carriers often limit the number of seats on each flight for which frequent-flyer awards can be used. You may not be able to get reservations on your first- or second-choice dates or flights.

* Awards can often be issued in the name of immediate family members. However, if you sell or give an award to someone not named on the award or the travel document and the airline finds out, the recipient could have his or her ticket confiscated, and the carrier may penalize the program member's account balance.

* Ask the airline how mileage is registered; you will probably have to identify yourself as a program member when you book your flight or when you check in.

* Keep your boarding passes and the passenger coupon of your ticket until you receive a statement from the frequent-flyer program reflecting the correct mileage earnings for that trip. If a problem arises, get the names of the people you speak with and keep notes of your conversations.