In terms of earning miles besides just flying a lot, Northwest WorldPerks has won awards by being among the easiest programs to earn miles.
As I said in an earlier post, I have had my account since April/2000. I have only flown 8,000 miles. I don't have the credit card, so I don't earn miles that way, but my total mileage balance is 43,000. NW offers bonus miles for practically anything. I've gotten them for using online check in, the E-Service Center at the airport, Connecting in MEM, renting cars, staying at hotels, eating out, grocery shopping.
Aside from all of the bonus miles, they offer miles on several different airlines as well.
NW gets my vote, but I am biased as well.