In the event that your iPhone is lost, and you need to attempt to find it using the "Find My iPhone" feature, make sure to have the setting for "Send Last Location" enabled. This setting will send out the last known location of the phone and will allow for you to find your phone even if the battery has died. Make sure your general Find My iPhone feature is already enabled.
You can follow these easy steps to enable the feature:
1. Go to Settings -> iCloud -> Find My iPhone
2. Flip the option for Send Last Location to ON
This feature is not a default so you need to make sure you manually do it. In a scary situation, this feature may be a real life saver.