I do as osewme does only I do it on my cutting table using giant binder clips to hold everything in place. I start with the center of the quilt - regardless of size. Clip all sides down then pin. When I have the center section pinned I slide the whole thing off to one side and repeat. Keep sliding it around on the table until I have the entire sandwich pinned.
Beats crawling around on the floor plus I don't have that much open floor space!