YOU'VE BEEN STEADILY earning frequent-flier miles for years, but lately your feet have been planted firmly on the ground. Be warned: Without account activity, you could lose those miles you've worked so hard to accumulate.

Happily, the situation isn't as dire as it sounds. "If you're not flying, you have a million options to keep earning miles," says Erica Silverstein, contributing editor and frequent-flier expert for SmarterTravel.com, a travel advice web site. "There's no excuse for letting your miles expire."

Keeping your miles can be as simple as ordering flowers for mom, signing up for your airline's newsletter or presenting your frequent-flier card the next time you rent a car. Here's what you need to know:

Check the Expiration Date
When and if your miles expire depends on the frequent-flier program, says Tim Winship, publisher of Frequent Flier.com, a web site devoted to frequent-flier information and advice.

Low-cost carriers like Southwest and Jet Blue tend to have programs that expire miles after a set period of time. "They simply extinguish the points after a year, and there's nothing you can do about it," he says. Most major carriers, however, including American Airlines, Delta, and U.S. Airways, will let you keep your miles indefinitely provided your account remains active. Typically, he says, your account would need to be inactive for three years before those miles would expire. (For details on specific airline requirements, see below.)

Staying Active
There are loads of ways to keep an account active, but here are our experts' favorites:

• Join Rewards Network

  For more on programs like this, see our column Earn More Miles

• Shop at your airline's online malls

  . Check your airline's frequent-flier program web site. Chances are, there's an online portal through which you can shop at your favorite retailers and earn miles for every dollar you spend. "You could go to Office Max and buy a pen, and keep your miles," says Silverstein. At Continental Airlines' ShopOnePass, you could get 10 miles per $1 spent at Godiva Chocolatier, or 1,000 miles if you subscribe to Blockbuster's online movie-rental service.

• Earn miles with monthly bills

  . Winship recommends finding one monthly bill for which you can earn miles. There are plenty of companies that offer miles with every bill, namely telephone and Internet services. T-Mobile, for example, offers up to 7,000 bonus American Airlines AAdvantage miles when you activate a new number and sign a one-year service agreement.

• Buy a magazine subscription

  . With some airlines, you can also keep your account active by redeeming (as opposed to accruing) miles. And one easy way to tap an account is through magazine subscriptions. They cost as little as 400 miles, says Winship, and depending on the airline you're almost sure to find a title you like. He likes American Airlines' AAdvantage program, which offers more than 25 titles.

Airline Guide
Each major airline has dozens of ways to earn or redeem miles and keep accounts active. For all your options, visit the frequent-flier section of the airline's web site. (URLs are provided below.)

Here are details on the expiration policies of the major U.S. airlines, plus an easy mile-earning option for each program: