I have a large number of miles on an airline which I rarely fly on anymore. I keep those miles "alive" by "trickle charging" them. Every time that I rent a car from Avis my account with Delta gets a hit. My 600K miles stay alive. One day I will use them for F tickets to SXM, but for now I'm draining my account with a different airline.

So, to keep miles alive:

1) Fly
2) "Trickle charge" with partners, rental cars, hotels, etc.
3) Online shopping thru airline website
4) Buy miles

1, 2, and 3 are good. Never do 4.