Originally Posted by
MSPeconomist
However, in some cases, it can be expensive to keep miles from expiring. Moreover, some foreign carriers expire all miles that haven't been used (in about three years, for example, on SQ) with no way to extend their validity (other than purchasing or transferring in enough miles for some sort of award ticket to use the miles).
Keep miles in the each main alliance partner US airline program. You can still keep them active pretty easily with little purchases, mileage donations, car rents, etc.