I'm no expert in either, but I do find it much easier to look at another person's outfit and see what it needs. When I look in the mirror I tend to stare and say "Duhhhhh, I know it needs something but what? Or does it need anything? Duhhhhh."
I'm sure it's because we have so much emotion tied up with our own reflection and are not objective. Taking a pic helps, but I'm still mystified most of the time.
Any tips appreciated!