When you find yourself in and out of subway stations and out and about in the city of New York, I bet that your smart phone quickly runs out of battery. Mine sure does and for that, I highly recommend carrying a lipstick size battery pack. The exact one is from Anker called the Astro Mini 3200mAh. It packs a powerful punch and is almost unnoticeable in your jacket pocket. It's also the #1 rater on Amazon for Camera Flash Battery Packs.
Pair that with a NomadKey Lightning to USB and you have yourself a perfect little pocket charging device. Take my word, you'll thank me later. For the Android or Windows Mobile folds, there's also a USB Mini variant of the NomadKey. I have both! =)