$40 mile retention fee? Has anyone done it?
 

what's the best/ cheapest way to keep miles active without actually flying US?

1. I did it on US Airways a few (several) years ago when it was $25. I had a large amount of miles and I was right at the deadline before they would had expired.

2. On AA, I donated 1000 miles to charity, the deduction posted quickly and extended things for 18 months.

For the above, I didn't fly much and got very busy.

I am not sure if US had a charity donation like AA does. I would post on the US Airlines thread and the pros there may give you some ideas on what to do.

You can donate miles, redeem for a magazine subscription, earn miles from a hotel, rental car, sending flowers, dining out etc. You just need any transaction on your account to keep them from expiring, so there are a few ideas. :)

It varies a bit by airline. My father needed to do this last year to keep his (iirc) USAir milage active. Rather than just paying the fee directly to the airline, there was an option to buy a gift card for one of their partner retailers & you got miles for the purchase. I think he bought a $50 gift card, kept his miles alive & gave my mom the card to buy whatever she wanted. Ironically I think he used his CO cc to make the purchase ;D

Buy a $5 Barnes and Noble gift card through the US Airways Shopping Mall.
 

Thats how I retained my US Airways miles in February.

Not only did I retain my miles but I got a gift card worth $5.

I recently purchased a book from Barnes & Noble through the USAir Mall for my younger daughter's account (a book I would have purchased anyway), and I bought a $10 gift card from Home Depot for my other daughter this weekend.

The B&N took 11 days to post (10 days to spare...) and I have a month to go for my other.

Dividend Dining Miles! Depends on where you live but it's super easy if you're in a big city

http://usairways.rewardsnetwork.com/