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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.
Turn off auto-brightness; adjust manually
Some believe that the iPhone's "auto-brightness" feature will help conserve battery life by dynamically and automatically increasing or reducing the brightness of the screen, based on how light it is around you. Yet, others argue that this alone can actually lead to the battery draining. It may not be easy to read your iPhone's screen in direct sunlight, and you may not get the most out of your high-resolution display, but this is about conserving your battery rather than anything else.
The best practice seems to point to disabling the auto-brightness. Go to Settings > Wallpapers & Brightness, then reduce the brightness to 10-25 percent, or whatever feels comfortable.