How long do frequent flier miles last? Unfortunately, not long enough. Among U.S. airlines, only Delta and JetBlue promise that your miles will never expire. A few hotel chains also offer no-expiration loyalty program points, but most airlines and hotels have timeframes that essentially force you to use your miles or forfeit their value.But there are a few tricks you can employ to extend those expiration dates with little-to-no-cost. In fact, with a bit of planning, keeping miles alive is so easy that there's almost never an excuse to let a single one expire.See: Best Airline Rewards ProgramsIf you have multiple loyalty accounts with different airline and hotel programs, it may seem challenging to keep track of all the varying expiration dates. But you can use a tool called AwardWallet to help organize all of them. This site will not only track the miles in all your loyalty accounts, but it will also keep tabs on your expiration dates and even warn you via e-mail when an expiration date is approaching. You can sign up for a free basic account on the AwardWallet site, but if you want more advanced features, such as an ongoing history of your mileage balances or the option to export the data to Microsoft Excel, you'll have to pay.Now that you know when your miles are going to expire, how can maintain their value? To keep your miles alive, most loyalty programs require that you have some sort of activity in your mileage account every 18 to 24 months. The good news is that this clock can be constantly reset. Each time you either spend miles or earn them, you start that 18 to 24 month period all over again, not just on any new miles earned, but on all the miles in your loyalty account.People often assume "account activity" is restricted to travel, but just a single mile of activity in your mileage account will restart the clock. Hotels and airlines have vastly expanded the number of ways you can acquire or use miles or points aside from flying or staying at a hotel, which means there are plenty of ways to earn and burn miles. The easiest way to find a list of partners for any airline or hotel is to go to its website and explore its loyalty program, where you'll inevitably find resources on how you can both earn and redeem miles. For instance, if the 18-month expiration deadline is approaching on your American Airlines AAdvantage miles, a quick trip to the airline's website reveals a number of retail partners with whom you can earn AAdvantage miles, including flower delivery services, DirecTV and a half-dozen rental car companies. Or, if you wanted to spend a few miles, you could buy gift cards to use at popular retailers, such as Macy's or Sears, purchase a magazine subscription or even donate your miles to charity.Remember that most of these redemptions are not the best use of your miles, as you can get much greater value by using them for travel. Your only goal with these particular redemptions is to extend the expiration date. Since any activity in your mileage account will reset the clock, you're likely better off spending just 300 miles on a 12-month subscription to Travel + Leisure instead of thousands of miles on retail gift cards.See: Shop Your Way to Free Flights and Travel PerksShopping portals are another way to create activity in your mileage account. If you've got an online purchase to make at a mainstream retailer, consider doing it through an airline or hotel shopping portal. You'll not only pick up additional miles, but also create activity in your mileage account, all without spending a penny more than you would have otherwise.The same is true of dining programs, which are partnerships between major U.S. airlines and restaurants that reward you with extra loyalty miles when you dine out. By signing up for these programs and linking your credit cards to them, you'll find yourself earning extra miles without any additional effort and pushing those expiration dates further down the road every time you grab a meal.To circle back to the previous example, if you were concerned about keeping your AAdvantage miles from expiring, you could apply for an American Airlines-affiliated credit card. For each month you spent money on the card and earned miles, the expiration date on all your AAdvantage miles would reset.Or, if you participate in one of the bank rewards programs that allow direct transfers to airline and hotel loyalty programs, such as Chase Ultimate Rewards or American Express Membership Rewards, a transfer of points to your airline or hotel account is also considered activity. Although you usually have to transfer a minimum of 1,000 points, in some cases it's possible to transfer as little as one mile from these programs, meaning just one mile could extend the expiration date on all your miles by more than a year.If all else fails, it is possible to transfer points and miles from one loyalty program to another using Points.com, which is an exchange for loyalty currencies where people can buy, sell and trade airline and hotel points. The transfer rates are terrible, so it's not advisable to do this on a regular basis. But if you've exhausted all other options and are desperate to keep your miles alive, it's better to lose a few miles by transferring them versus losing a ton of miles by letting them expire. If you do choose this option, keep the number of points you exchange to a minimum.As a last resort, you can either buy miles directly from the airline or pay to get your miles reactivated after they've expired. In certain extreme cases, these options may be worthwhile, but both of these methods will likely cost you more than the miles are worth.See: 5 Ways to Earn Miles Without FlyingAbout the author: Julian Mark Kheel learned the ins and outs of travel loyalty programs while flying more than 200,000 miles a year as a TV producer and director. He takes a contrarian view on travel wisdom in his "Devil's Advocate" series every Thursday at the blog Travel Codex. You can also reach him on Twitter @dvlsadvcate.