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Disable Siri's 'raise to speak' feature
Siri can be activated by holding your iPhone close to your face, as though you're having a face-to-face talk with the intelligent assistant. But the proximity hardware sensor is always checking to see if you are nearby. This may impact the battery over the course of the day.
Head to Settings > General > Siri, then disable the Raise to Speak button to disable the feature. The alternative isn't so bad: You can still hold down the home button to speak to Siri.
Prevent Spotlight Search from indexing unnecessary files
Spotlight needs to index your entire phone's contents regularly so that it can serve up the best results. This may not use up a whole lot of processing power, but it's certainly helpful in the long run by removing items from the list that you don't use.
You can select which areas of your phone to index by heading to Settings > General > Spotlight Search and tapping any item you wish to exclude from the search index.
Tame your notifications (and your backlit display)
Notifications alone don't cause battery drainage, but the sheer act of activating the backlit display will. Applications use notifications to inform you of what's going on in the world, such as new emails, text messages, reminders, and who is responding to you on social networks. It's wise to set some notifications to "silent" mode, but this requires some tinkering about in the settings.
Head to Settings > Notification Center, then scroll down. Under each category of notifications that you would like to mute, such as low-priority features like Photos or Game Center, tap through to each setting. Under Alert Style, simply tap None (the left-most graphic). Repeat for each setting.