American Airlines Mileage donation program here: http://joinus.aa.com/miles-for-kids-in-need-aadvantage-miles-donate is both a great cause and a good way to make sure your miles don’t expire unused. Donations will extend your expiration date – usually to 18 months from the date of donation. The minimum donation is only 1000 miles so this is probably the “cheapest” way to extend miles as the donation only “costs” you 1000 miles – a value of about $10-$15 depending on your flying and mileage program habits. My personal rule of thumb *used to be* to use money when the cost of the ticket was LESS THAN a penny per mile used, but this has been complicated by the fact that I have a lot of miles now from credit card offers, ticket have become very expensive, miles tickets are HARD to get, etc. I’m now inclined use miles whenever possible simply to avoid losing them.
In any case, be SURE to check your miles expiration if you have more than about 5000 miles in your account. Fewer than that and it may not be worth the time to mess with miles now that tickets have become hard to get using these programs. For those short on time and long on money I’d say under 10,000 miles may not be worth your hassle time, but obviously if you have over 25000 miles – basically a free round trip in the USA on many airlines – you’ll want to preserve those miles.
As always, the secretary disavows any knowledge of your mission. Good Luck.