Rental-car companies now have clubs similar to frequent-flier clubs and have copied the airlines' system of constantly modifying prices and restrictions to maximize rentals.

Membership in Virgin Atlantic's frequent-flier club (free) is required; see www.virginatlantic.com.

But Mr. Britton noted that airlines are gaining market share by successfully wooing travelers who are not yet committed to a frequent-flier club.

Passengers can choose which frequent-flier club will get the miles for the trip.

Unlike Southwest, Calite offers assigned seating, not to mention a more elaborate frequent-flier club.

Previously, when an airline has folded, members of its frequent-flier club have been protected because its operations and routes were sold to another carrier.

And for the philanthropic, members of many frequent-flier clubs can contribute their certificates for free travel to a number of charities.

Among the first businesses to link themselves to frequent-flier clubs were travel-related companies.

And many carriers regularly auction vacation packages to members of their frequent-flier clubs, who submit bids of their miles.

"It is preferable to use a luggage tag with your business address or a tag given by a frequent-flier club with a name and an identity number only," he said.