I found a great tutorial once, link
the most important thing (from the text)...
"Start by sharpening the thing closest to you, which is known as your foreground. Then slowly fade your sharpening by lowering the strength as you go and sharpening the very back of your foreground with the lowered strength" (Same with blur, work your way from the bg to closer your render)
and.. "IMPORTANT!!! The above process should be done on an image applied layer" (make a new layer above all and do Ctrl+Alt+Shift+E)
Still, read everything cause thats's some good shit, there