Traffic on the nation's airlines has been picking up, but carriers are still offering fare sales and other inducements to keep the fragile recovery in travel moving ahead.
The latest example is Southwest Airlines Co., which said Wednesday it would give members of its frequent-flier program free travel by taking four roundtrip flights. That's twice as fast as fliers would normally earn a trip.
Southwest said customers who buy a higher-priced "Business Select" ticket will earn a trip even faster.
The offer covers travel through May 26, meaning it expires before the beginning of the peak summer travel season.
Frequent-flier programs date to the 1980s and have become an important tool for airlines to gain repeat business from travelers eager to earn free trips. That's increasingly true now, as airlines often match each other on basic fares, leaving customers to choose carriers based on mileage awards and other factors.
Some carriers have tried to boost the appeal of their loyalty programs by making it easier to use miles. For example, American recently began letting customers redeem miles for one-way travel, not just round trips. Many airlines also let customers use miles for purchases other than travel.