Not a lot will help that other than finding a way to keep the phone warm--it doesn't generate enough of its own heat that a case will help much--particularly since anything that lets you use the screen lets heat escape through it unabated. The reflective pouches like the Phoozy are definitely better than nothing but there's no substitute for keeping the phone as close to a heat source (eg your body) as possible. Even if it's only 10C/50F under your jacket, that's way better than freezing the phone outside.