United Airlines took another step on Wednesday to combine its frequent flier program with Continental's. It's boosting rewards for the most expensive tickets and adding an expiration date, which will be new for Continental fliers.

Most airline frequent flier programs were built on the idea of one mile of credit for every mile flown. That rewards distance over dollars.

In January, Southwest revamped its frequent flier program to give larger rewards depending on the fare. It said explicitly that it believes some customers will choose a slightly higher fare to get a better reward.

Now United is headed the same way. Travelers flying on first-class tickets will get up to two-and-a-half times the miles flown, and those using business class and full-fare coach tickets will get more miles, too.

The airline says it's trying to make those tickets - including full coach fares and front-of-the-plane first-class fares - more attractive.

"We absolutely have every intention of acknowledging and recognizing both the frequent traveler, as well as the travelers that are creating the most value (for the airline). We want to make sure that we're providing the right balance to the program to accomplish both of those tasks," said Jeff Foland, president of United's MileagePlus program, in an interview.

Miles in the combined program will expire after a year and a half. That's not a change for United travelers. Continental miles had no expiration date, although accounts could become inactive after a year and a half if no new miles were earned.

United Continental Holdings Inc., formed a year ago by the merger of the two airlines, is combining them into a single carrier flying under the United name.

The company's new program will have four "elite" tiers instead of three, with new levels beginning at 25,000 miles, 50,000, 75,000 and 100,000.

Even as airlines try to reward big-spending travelers, many fliers are deciding that frequent flier miles aren't as valuable as they used to be. Their value has been hurt by expirations, more miles being needed for a free ticket and limited availability of seats.

Foland said accessibility of reward seats should stay about the same under the combined program. The combined program has 85 million members.

The company has said the combined frequent-flier program will be called MileagePlus - the name of United's plan.

Continental's OnePass program will end on Dec. 31, and members will later have their credits converted to the MileagePlus program. Continental frequent fliers will continue to get miles and upgrades between the end of the year and the conversion early next year, United said.

The changes announced Wednesday will begin when the two programs are combined early next year.