The more expensive and bulky yet universal solution is one of those portable wifi-connected hard drives that allow smartphone access thru an app (over direct or shared wifi). There'd a few dozen models, they'll work with Android, iOS, PCs, etc.
A cheaper but potentially more convenient (and decidedly more compact) solution for recent Android phones would be a 3-6" USB OTG cable with one of those thumbnail sized USB microSD readers attached at the end (the kind that only stick out like 5mm from a USB port). Costs like $5 altogether + the cost of the card (64GB is down to what, $30?) and acts just like internal storage if you get a fast card.
I'm surprised that kinda thing isn't more common already considering almost everyone is ditching removable storage yet USB OTG is now commonplace amongst recent phones, it's just like using a flash drive on a PC, almost sidesteps the issue altogether.
Edited by Impulse - 4/7/13 at 8:55pm