currently several of the airline shopping portals are offering additional bonus, United airlines offering spend $400 get additional 2500 miles, and AA offering spend $800 get 2000 miles.
虽然是蚊子肉, but they can add up fast, especially United, as you can pool the miles together, so family of 4, can earn 10000 miles on 1600 purchases, that can be a RT ticket in continental USA in the low season.