When you're doing big events and festivals like Jingle Ball or the iHeartRadio Pool Party, your lifeline is your phone. I always leave an iPhone cable in the top pocket of my laptop bag, but sometimes you don't get to plug in.
Enter the Mophie Juice Pack. A case for your phone with a battery built in.
image courtesy Mophie
The one above is the Juice Pack Air for iPhone 5; I have a Juice Pack Helium.
It's made of soft-touch plastic and features a micro-USB port (like the ones on Android phones) instead of a Thunderbolt or Dock connector. You charge it ahead of time and then just place it on standby with a switch on the bottom until you need the juice. When on, the Helium I have gives you enough power to recharge your iPhone battery to about 80%; the Air can give you pretty much a full charge.
The downsides: It adds some weight. It's only 2.5 ounces or so, but that little bit makes a iPhone 5 feel like a 4, so it's not ideal for everyday use. The price is a bit for some, at $80 for the Helium and $100 for the Air. It also adds some depth at the bottom, so you need an extension for the headphone jack if you want to use it (and usefully they include one with the case).
Still, pretty useful, and it'll keep you up and running long after everyone else in the crowd gets the sad drained battery notification. Keep it in your travel bag with your spare cord, and you're pretty much set.
Available via Mophie's online store.