A Portable Phone Charger
This shouldn't even have to be said, but we'll say it anyway: a portable phone charger is absolutely necessary. Looking for an outlet to charge your phone during fashion week is like looking for a needle in a haystack but the needle's already been found by someone else who didn't bother telling you and left you crawling on your knees through hay with all five of your senses horrifically assaulted by allergies. So just get one. It'll save you so much stress, let you actually do your job if you're not just attending the shows for fun, and enable you to snapchat story to your heart's content without any inhibition. Though the mophie Juice Pack is one of the pricier versions, it's quite light and allows for quick dual charging (and also is apparently an NYFW staple for our favorite Olivia Palermo).
mophie Juice Pack PowerStation External Battery, $79.95, Backcountry.com