American is the most generous about granting elite status to travelers who lack the requisite miles. Periodic promotions boost customers who agree to fly an extra 5,000 or 10,000 miles in 90 days, even on discounted fares.

Key markets: Chicago, Dallas, Los Angeles, Miami, New York, San Juan

Elite miles: Gold (25,000), Platinum (50,000), Executive Platinum (100,000)

Pros: Members can lock in upgrades to business class on domestic flights early with $30 coupons worth 500 miles apiece. Admirals Club members have free access to Regus Group's fully equipped business lounges in 180 U.S. cities.

Cons: Unused miles expire after three years, and only Executive Platinum members can expect automatic upgrades. Members say in-flight service has declined and the new business-class seats don't compare to those on rival carriers.