Since we've become increasingly beholden to our phones, we can't get through an afternoon out and about without our phones dying around dinnertime. This is because we can't spend any downtime (waiting in line at a store, waiting for the subway, waiting for a friend in a bar) not playing on our phones. All of this excessive usage obviously drains your battery. The days of having your flip phone's battery last for 4 days without a charge are gone.
So, if you don't want to be that person begging your waitress at dinner to plug your phone in behind the bar (trust me, you don't want to be that person), you have two options:
1.) Deal with a dead phone. You won't be able to get in contact with anyone, but think about the flip side—no one will be able to get in contact with you. Freedom, sweet, glorious FREEDOM.
2.) Invest in a portable charger that doesn't require a power source. There are a lot to choose from. Mophie's are the most common, and they work well, but the problem with them is that they make your sleek, beautiful iPhone weigh the equivalent of a brick. You'll break your wrist trying to make a phone call with that thing. That's why I bought this On-the-Go Charger ($28, Jonathan Adler) instead. It's light-weight and fits in the palm of your hand. You charge it up before you leave the house, throw it in your bag, and when your phone's battery is getting low, you plug it in and it'll give you up to a 70% charge. They also have a different one for the iPhone 5 (for $38). You'll never be without Instagram again.