A word of warning: layer styles can only contain integer values. If you edit a
drop shadow offset to be 1 px with the document at 2× size, and then scale
it down, the value will end up as 1 px because it can’t be 0.5 px (a non-
integer value). If you do require specific changes to the 2× version of the
Photoshop file, you’ll have to save that version as a separate file.
Exporting, Exporting, Exporting
Now for some bad news: exporting all the images to build an app can be
extremely tedious, and I don’t have much advice here to assist you. As my
documents act as full screen mockups, they’re not set up in a way that
Photoshop’s Slice feature is any use. Layer comps don’t help either — I
already have folders for each app state or screen, so switching things off
and on is easy.
The best export method seems to be: enable the layers you’d like visible,
make a marquee selection of the element, then use Copy Merged and paste
the selection into a new document — not much fun when you have
hundreds of images to export.
The problem is amplified when saving for the Retina display, where there
are twice as many images and the 1× images must match the 2× images
precisely.