The simplest way to keep miles from expiring to to buy something that gives you miles for your purchase. Usually flower shops like FTD.com or 1800flowers.com do that. Or order something with your miles, like a magazine subscription. all you need to do is "touch" your miles usually once every 18 months although times vary by airline.
I agree with Dan, A"A by a longshot. I basically fly only A"A now. I have 180,000 miles with them (I don't churn cards).
