When you have misc. miles on airlines you have not used in the last three years, but you may use in the future or you may wish to transfer at a future date, most airlines have a policy of wiping them out.
How do you protect against this outside of flying?
we were told that changing the address or phone number was considered by some airlines as "activity".

Sign up for Idine on your card and eat one meal in an Idine restaurant.
Order something from the shopping mart for that particular airline.

Send your sweetheart flowers, you'll get "bonus" points.

Donate a some miles to the relief effort in the Gulf states.

Get a starwood Card and move a mile or two to your program.

Shop online through the airline / hotel portals, donate points to hurricane katrina relief, get a credit or debit card from the airline, redeem other loyalty points for miles, buy a minimum amount of miles from the airline or through miles4sale etc etc etc

See: http://www.andrewcram.com/frequentflyer.html for reference tables that cover most of these options.

Can you really move as little as one mile? I take it that's from the card, and not regular Starpoints.

I tried to look up whether one can transfer a single Starpoint, but the www.spg.com website is down right now.

Diners Club also works well at keeping accounts alive, albeit in 1000 mile increments.

Only SPG Platinum members can transfer 1 point. For Golds, the minimum is 1500 & for basic members, 2500.

Also, all points earned on the SPG card are regular *points.

Stay at one of their partner hotels or rent one of their partner car rentals, and chose their "bonus" miles.

Shop through their mileage mall and get something inexpensive. An example is buy at Circuit City a package of batteries. With in-store pickup, there will be no shipping charge. So for $3, you may get 9 bonus miles - which will keep your account active for another 3 years! (All 10,000 or 150,000 miles!)

Or any Amex card should do the trick!!

"Account activity" for this purpose generally means anything that affects your mileage balance, up or down, by any amount. Things like address changes are generally not "activity." While each airline writes its own rules and can define activity any way it wants to, it would be too easy for people to extend the lifetime of their miles indefinitely if all they had to do was create a minor variation of their address every 35 months or so.

Several suggestions for ways to earn miles have been made in previous posts. Check your airline's Web site or the link posted by ANDREWCX for its list of partners. Some of the suggestions made above work for many airlines but not all of them, and the OP didn't say which airline(s) s/he is interested in.

That's not a bad idea at all. ^

I have used IDine the most in this way since it could have a $5 meal and it will count, and you can easily change your credit card back and then back again to your favorite airline - if you don't have enough different credit cards to go around.

I like the AA kelloggs promotion, Delta Coke a Cola, AA Dannon water, and various games and puzzles that the programs come out with occasionally. They only give you a few hundred miles but they are cheap or free to participate in. Check Pgary's web site or the airline for current promotions.

Many rental cars will give you 50 miles.

Another thought - check your airline's definition of "three years." With AA, it's 36 months to the day after the last activity. With DL, it's December 31 after that, giving a little extra time to extend their life with something you might have done anyway. (Just found out about this yesterday when I saw that a recent AF flight extended my DL miles to December 31, 2008. I don't fly DL enough to have regular activity there, but I have a large enough balance with them to care about not losing it.)