Staples work, roofing nails too.
the stuff just sucks, and lord help you if you have to flash them a bit as sometimes they will just float off the pallet after a flash.
lots and lots' of glue, flash mist type adhesive helps a bit, but it's still not 100% good. We tend to build a base of web adhesive and then go flash/mist on top, but sometimes 4-5 coats of them while the platens heat up prior to printing on the auto, then flash/mist adh. before every new shirt.