WHEN IT COMES TO

airline mileage programs, it's nearly as rewarding to be a frequent buyer as it is to be a frequent flier.

But if you aren't steadily racking up miles through either method, those miles could be in jeopardy of expiring. As of January, several airlines have new, quicker expiration dates in place for miles:

United Airlines Mileage Plus miles expire after just 18 months of account inactivity. (Previously, they expired if you hadn't dusted off the account balance within three years.)

Delta SkyMiles expire after two years of account inactivity, instead of three.

U.S. Airways Dividend Miles expire after 18 months of account inactivity. Before the January change was put in place, consumers had three years.

If you're willing to spend either cash or miles, there are plenty of mile-preserving tactics you might use. But if you'd rather save both for use on an approaching vacation, try these four free options to keep your account and its miles intact:

1. Take a Survey

When planning a leisure trip, what's the first arrangement you make hotel, airfare, rental car, leisure activities or none of the above? Take five seconds to answer Delta's one-question

survey

, and earn 250 bonus SkyMiles. (This deal is only available to members who received Delta's winter e-newsletter.)

To snag survey opportunities, pay attention to your monthly frequent-flier activity statements and deal newsletters. But you'll need to look closely, cautions Stephanie Abrams, host of the Business Talk Radio Network show "Travel with Stephanie Abrams." Survey mentions don't always advertise that miles will be awarded. And when they do, the offers are buried among miles-with-purchase promotions for airline partners.

2. Go Paperless

"Airlines love to use bonus miles as a carrot to entice people onto the web," says Tim Winship, publisher of

FrequentFlier.com

. "It's an incentive that helps them decrease costs." American Airlines, for example, awards 1,000 bonus AAdvantage miles when you sign up to have statements delivered by email rather than snail mail. Log in to your online account, and you'll find these deals front and center, he says, under limited or promotional offers.

3. Share

New rewards program

e-Miles

, which launched in November, has partnered with both airlines (Continental, Delta, Northwest and US Airways) and market research companies. Sign up for a free account, and you'll earn miles from the former as guinea pigs for the latter watching ads, completing surveys and filling out questionnaires about your buying preferences. You'll earn 25 miles, for example, by answering two questions about what kind of electronics you own. Miles are deposited quarterly, in 500-mile increments, to the frequent-flier account of your choice.

4. Exercise Purchase Power

Airlines are rapidly expanding their online shopping portals, including more mainstream retailers, says Winship. While you shouldn't buy just to earn miles, you can come out ahead by checking the portal for offers before making purchases you would have made anyway. Bookmark your airline's online shopping portal, and use it to link to your favorite retailers. A contact-lens wearing Continental OnePass member could get five miles per $1 spent, plus free shipping, for filling a prescription at 1-800-CONTACTS. "Those are truly free miles," he says. "No additional money or energy was expended."

Not every offer is web-based, either. Enrolling your credit card online can earn you miles for dining out, buying groceries and making other everyday purchases. United, for example, awards 125 miles for every $250 grocery purchase at chains including Safeway, Genuardi's and Vons.