Somedays it seems like all you have to do is take your iPhone off the charger and it's already dipped below 90%. Carrying a Mophie or wireless charger around is a simple fix, but there are ways to see what's actually killing the life of your iPhone's battery.
Once you open up Battery Usage [see below], you'll be able to view a list of apps in order of battery consumption. There's a 24-hour view and a view for the last seven days, which is handy whether you're trying to explain a sudden change in battery life or if you're just taking a look at your habits over time. It's an overdue addition to iOS, but an extremely useful one, assuming you know it's there at all.