Back in '07 (before the NW merger and the economy tanked) I received a promo for double miles on every flight for 3 months after I had stopped traveling DL. At the time I was involved in a contentious situation down in N.C. and was traveling back and forth at least once a week (sometimes more often). The situation resolved itself and as it turned out didn't need to travel or wasn't on DL for over 6 weeks--although I bet I credited a CO trip or two. Then bang, I got a "we really miss you" email and an offer for double miles. It was a great deal b/c it had almost no strings and I started flying BOS/JFK-LAX on a weekly basis soon thereafter. We took a very nice family vacation in '08.

Somehow I feel they don't do that anymore. Frankly the game has changed. The goal is less about finding and keeping FF customers and more about changing the customer behavior now (i.e. making up buy more expensive tickets).