Most airlines have some form of expiring miles. Most programs promote that their miles never expire… but there's a catch. The miles may not expire, but the account itself can become dormant with all miles forfeit if there's no activity for a prolonged period of time.
Only Delta, among major frequent flyer programs, offers no mileage expiration – period. Ironically in the middle of the last decade they were one of the early adopters of shortened expiration times. Most US frequent flyer programs expired miles after three years, and they led the charge to reduce that initially to 24 months and they even limited the types of activity that would extend the life of an account. Once they had expired a bunch of miles off of their balance sheet, they announced their miles would no longer expire. Though they promoted it as 'the right thing to do' they didn't go back and restore miles they had taken away.
Nonetheless, I don't mind expiring miles as long as the policies are clear and the rules aren't draconian like Spirit Air's program which expires miles if you do not earn miles in your account every three months.
You need to keep your account active, be an engaged member at some level, and your miles will be fine. Use a tracking tool that helps you manage your frequent flyer programs and track expirations. I use Award Wallet.
Once you have your balances, and many of your account expiration dates in front of you, it's easy to keep miles alive in most frequent flyer progams.
You'll see that some non-US frequent flyer programs will expire your miles no matter what you do. You have to use miles earned right away, or least within three years, or else you will lose them. Then there are hybrids, like the Air Canada Aeroplan program, where you have to keep your account active with some sort of earning or burning every 12 months or else you'll lose your miles. But all miles earned, regardless activity, will lapse after 7 years.
So what are the simplest ways to keep an account active? The particulars vary by airline, since their specific partners aren't all the same, but in general the tools to keep at your disposal (in addition to flying the airline, or using their co-branded credit card) are:
Tim Winship used to sign off, "May your miles never expire!" Use these Hunger Games tricks and the expiration odds will be forever in your favor!