Photoshop :: Bending And Blending Shadows, Dual Surface

i'm trying to cast a shadow, behind this polaroidy picture, that makes it look like its resting against any imaginary vertical surface and setting on an imaginary horizontal surface. The quality of my current shadow should give you some idea of how long ive been using photoshop

I've been working at it a little while and i cant seem to get anywhere. I have the feeling the trick lies somewhere in the confluence of two gradient shadows, coming together and bending to form a slightly darker area where the imaginary 90 degree angle would be.