Delta is offering double base miles on all flights between August 1 and October 15, but you have to register. What are “double base miles?” Since you asked… it normally takes flying 25,000 to become an elite flyer (which makes you eligible for first class upgrades and lots of other good things). Double base miles means that each mile you fly counts as two. Fly 12,500 miles and become an elite. Reach the next level at 25,000 instead of 50,000, too!