Even though she's got me k/f'd I still think she might check out sheet
straps (aka suspenders). KMart and other stores like it sell elastic straps
that fit on the corners of the bottom sheet. They fasten like old-fashioned
girdle fasteners. They hold the sheets tight to the corners on large
mattresses. They're a bit of a PITA but then again, so are the sheets
themselves Just a suggestion.

Jill