If you let your airline miles expire (they typically expire within 18-36 months), you're doing it wrong. Every airline I'm familiar with will reset the expiration clock (aside from Delta, which doesn't have one) if you earn even a single mile, and they're easier to earn than you'd think.
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Use Dining Rewards Networks
When you join a dining rewards network you provide your credit card number(s). Whenever you use a linked credit card at a restaurant within the dining network (they all provide maps of restaurants in the network) then you will earn miles proportional to your purchase amount.
Most (all?) airlines have dining rewards networks, such as this one from Southwest. Google <airline> dining rewards network and you should find it.
Note: Each credit card can only be linked to one airline/hotel dining rewards network at a time. If you try to link it to more than one, you will eventually get a notification that your card has been unenrolled from the previous one.
Use Shopping Portals
A shopping portal is a site that has affiliate links to most major online stores. When you use their link, the shopping portal gives you a portion of the revenue.
While there are plenty of cashback portals such as TopCashBack (referral link) or eBates (referral link), there are also airline and hotel miles/points portals such as Southwest Rapid Rewards Shopping or American Airlines AAdvantage eShopping. Instead of earning cashback, you earn miles/points proportional to your purchase amount.
Credit Miles to a Partner Airline
If you happen to be flying on say British Airways, but your American Airlines miles are expiring, credit your miles for the British Airways flight to American Airlines. You can do this for most (all?) airline partners.
I really don't like these options, particularly when the first several options are easy enough.
- Purchase gift cards or a magazine subscription with your miles
- Purchase miles
Other Methods to Reset the Clock
I'm lumping these all in here because if you're reading this post, you've most likely already thought of these options. I leave them here for the sake of completeness.
- Transfer miles from a transferrable points credit card. For example, transfer Chase Ultimate Rewards (UR) points to United.
- Use the airline/hotel specific credit card, such as the Hilton Amex.
- Book a flight (though this won't work on Southwest, because they only reset it for earning activity)