American Airlines tosses out all your miles if you haven't flown with them for 18 months straight. So Jaunted's very own markj called up AA and donated some miles to charity. The account activity will keep his miles active, and he gets warm Xmas fuzzies for it, too.
Continental miles don't expire, so you don't have to worry about this business with them! (Still, if you're feeling gifty, you can donate to charity with your OnePass account.)
Delta miles expire after just 12 months, so as was the case with JetBlue, you might wanna check on this pronto. Not to worry, though: Delta has lots of partnerships that can keep you alive. Buying flowers for the holidays can earn you some points, as Delta has a deal with FTD and 1-800-Flowers. You can also keep your account active by booking a reward trip, if you've already got enough points.
Northwest has what they call the WorldPerks Mall, which is basically like Amazon--with the bonus that you earn miles when you spend money. The problem here, though, is that it may take six to eight weeks to get your mileage credit. Not exactly advisable if you're close to your deadline.
United also has a charity program, and you can donate online or by phone. If you're feeling like you deserve the gift this Christmas, stays at Hilton, Hyatt and InterContinental hotels net you miles without hopping on a plane.
US Airways, like NWA, has an online mall, and they even have a deal with iTunes. So theoretically you could buy a single track for 99 cents and keep your miles alive for another 18 months. A single, solitary song? Better than a mileage run on US Air!
There's no way we can cover all the bases here, so you might check your own particular program for details. Generally any account activity will keep your points intact, so you don't have to fly--unless of course you want to.