There is a special kind of frustration in discovering that thousands of miles you earned have expired because you forgot about them. Airline miles are not like money in a bank: many programs delete your balance after a period of inactivity, and the rules vary wildly. The good news is that keeping miles alive is usually easy, and often costs nothing at all.

Some airline programs never expire miles, including Delta SkyMiles, United MileagePlus and JetBlue TrueBlue, which is one reason they are popular with casual travellers. Many others expire miles after 18 to 36 months of inactivity, meaning any earning or redeeming activity resets the clock. A few programs expire miles on a fixed date regardless of activity, which is much harsher and requires real attention.

Hotel points usually expire after 12 to 24 months of inactivity, and bank points generally do not expire as long as your account is open. The first step is to know the rules for each program you use. Write down the expiry policy alongside each balance, and set a calendar reminder a few months before any deadline so you are never caught by surprise.

In most programs, any activity resets the expiry timer, and the easiest activity is often free. Earning a single mile through a shopping portal, a dining program, or a survey is usually enough. Some travellers buy a small number of miles, though that costs money and should be a last resort. Transferring a small number of points from a bank program also counts as activity in many programs.

Credit cards tied to an airline program often keep miles alive automatically, since every purchase earns miles and counts as activity. If you hold such a card and use it even occasionally, expiry is rarely a concern. The travellers who lose miles are usually those who earned a balance years ago and then forgot the program existed entirely.

The most reliable protection is a simple tracking system. Keep a spreadsheet or use an award-tracking app listing every program, the balance, the expiry rule, and the date of your last activity. Review it every three or four months, which takes only a few minutes, and take action on any balance approaching expiry.

Also consider consolidating your earning. Holding small balances in ten different programs is far riskier than holding a large balance in two, because small balances are easy to forget and hard to use. Focus your flying and spending on one or two programs, keep the rest alive with occasional small activity, and your miles will be there when you need them.

It is also worth checking whether your miles can be kept alive by donating a small number to charity, which some programs allow, or by buying something small through the airline's shopping portal, such as a magazine subscription or a low-cost item from a partner store. These options cost very little and count as activity in most programs. Another often-overlooked option is to transfer a small number of hotel points into an airline program, or the other way around, where the programs are partners. Even a transfer of a thousand points can reset the clock, and it keeps your balances useful rather than letting them quietly disappear. The cost of any of these actions is tiny compared with the value of the miles they protect.

Miles expire when you forget them, not when you use them. Know each program's rules, reset the clock with a small activity every year or two, and track your balances in one place. A few minutes of attention protects thousands of miles.