Delta is offering double miles on mileage transfers between Skymiles accounts from October 1 through October 31.

They seem to be worried about folks creating fake new accounts in order to transfer miles back and forth to themselves, so they’ve added a little clause into the rules that Skymiles accounts must have been open 60 days and have earned some miles in the past in order to be eligible for the bonus.

Of course adding such things into the rules just points out the obvious, that if you’re looking to purchase miles inexpensively this is a great way. For example,

• Transferring 30,000 miles will cost $330, and yield 60,000 miles.

• Transfer 30,000 miles back to your friend for $330 and they receive 60,000 miles.

• On net you’ve created 60,000 bonus miles for $660 or 1.1 cents apiece. (You each get 30,000 miles for $330.)

If you have friends with orphan miles, or family members, this is a great way to combine their balance into yours.

What’s the most you can do under this bonus? Each account can receive up to 300,000 miles per year and transfer out 150,000 miles per year.

Granted, Skymiles aren’t my favorite currentcy. But buying large amounts of Skymiles cheap can be a good way to redeem for Singapore Airlines first class awards (with the caveat that they’ll also charge you exorbitant taxes and fees). Still, my advice is only to load up on the miles you actually plan to use in the very near term rather than trusting the Skymiles program to retain the value of your miles over time.