I love slingbacks too, and buy them all the time even though I can't keep the backs up! Any tips out there? it is very frustrating! I have even tried shortening them and they still won't stay up!
Two-sided tape, trimmed to fit and placed discretely in the center of the strap. Press it onto your heel and VOILA! No more slipping. Just don't put any cream on your feet though, or it won't stick.
What brand are the slingbacks at the very top?