What's the best way to model organic shapes? For eg: I have this sandal I'm trying to get the edges to curl upward slightly as well as the whole toe (as seen in the insert). Try as I have over and over -- trying to deform the mesh shape using proportional editing is just not cutting it! 

Any suggestions?
The current topology cannot benefit that. The basic workflow and recommended topology would be:


Create the base mesh for the sole.
Add a Subdivide Surface modifier, then apply.
Add another Subdivide Surface modifier. Tweak mesh shape with Proportional Editing.
Build the straps with operations such as Extrude, Bridge and Loop Cut.
Build other details, such as thickness.
Finally refine the shape.




Example file: